SELF-STANDING GaN SUBSTRATE, GaN CRYSTAL, METHOD FOR PRODUCING GaN SINGLE CRYSTAL, AND METHOD FOR PRODUCING SEMICONDUCTOR DEVICE

ABSTRACT

An object is to provide a nonpolar or semipolar GaN substrate having improved size and crystal quality. A self-standing GaN substrate has an angle between the normal of the principal surface and an m-axis of 0 degrees or more and 20 degrees or less, wherein: the size of the projected image in a c-axis direction when the principal surface is vertically projected on an M-plane is 10 mm or more; and when an a-axis length is measured on an intersection line between the principal surface and an A-plane, a low distortion section with a section length of 6 mm or more and with an a-axis length variation within the section of 10.0×10 −5  Å or less is observed.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of International Application PCT/JP2014/070919,filed on Aug. 7, 2014, and designated the U.S., (and claims priorityfrom Japanese Patent Application 2013-165293 which was filed on Aug. 8,2013, Japanese Patent Application 2014-006907 which was filed on Jan.17, 2014, Japanese Patent Application 2014-038722 which was filed onFeb. 28, 2014, Japanese Patent Application 2014-090535 which was filedon Apr. 24, 2014, Japanese Patent Application 2014-104383 which wasfiled on May 20, 2014, and Japanese Patent Application 2014-122520 whichwas filed on Jun. 13, 2014) the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention mainly relates to a nonpolar or semipolarself-standing GaN (gallium nitride) substrate.

A bar-shaped nonpolar or semipolar GaN substrate can be obtained bygrowing a GaN crystal along a c-axis on a 2 inch-diameter C-plane GaNtemplate using an HVPE (Hydride Vapor Phase Epitaxy) method and slicingthe GaN crystal so that a nonpolar or semipolar face becomes a principalsurface (Patent Document 1). Furthermore, a plate-like nonpolar orsemipolar GaN substrate can be fabricated from a GaN crystal vapor phasegrown using a plurality of the bar-shaped GaN substrate arranged side byside as a seed (Patent Documents 1 to 3).

A method of fabricating an M-plane GaN substrate by slicing a hexagonalprism shape GaN crystal grown by a flux method and a method of using theM-plane GaN substrate as a seed to grow a bulk GaN crystal by an HVPEmethod and fabricating an M-plane GaN substrate by slicing the bulk GaNcrystal are proposed (Patent Document 4). According to an HVPE method, aGaN crystal can be grown at a rate 100 times faster than a flux method.

A self-standing GaN substrate obtained by separating a GaN crystal grownby an HVPE method on a C-plane sapphire substrate on which a “voidbuffer layer” is formed from the sapphire substrate has been reported tohave an a-axis length (also referred to as an a-axis lattice constant,an a-plane spacing, and the like) and a c-axis length (also referred toas a c-axis lattice constant, a c-plane spacing, and the like) of 3.189Å and 5.185 Å, respectively (Non-patent Document 1).

It has been reported that, after subjecting a GaN crystal grown by anHVPE method on a C-plane GaN/sapphire template and having a c-axisdirection size of 3.5 mm to heat treatment at 1300° C. for 6 hours, ana-axis length (an A-plane spacing) thereof was measured to be within arange of 3.18915 to 3.18920 Å over 3 mm in a c-axis direction (PatentDocument 5).

-   [Patent Document 1] Japanese Patent Application Laid-open No.    2006-315947-   [Patent Document 2] Japanese Patent Application Laid-open No.    2008-143772-   [Patent Document 3] Japanese Patent Application Laid-open No.    2011-26181-   [Patent Document 4] Japanese Patent Application Laid-open No.    2008-110910-   [Patent Document 5] Japanese Patent Application Laid-open No.    2012-231103-   [Non-patent Document 1] Hyun-Jae Lee, S. W. Lee, H. Goto, Sang-Hyun    Lee, Hyo-Jong Lee, J. S. Ha, Takenari Goto, M. W. Cho, and T. Yao,    Applied Physics Letters 91, 192108 (2007)-   [Non-patent Document 2] Po Shan Hsu, Matthew T. Hardy, Erin C.    Young, Alexey E. Romanov, Steven P. DenBaars, Shuji Nakamura, and    James S. Speck, Applied Physics Letters 100, 171917 (2012)

DISCLOSURE OF THE INVENTION

Improvements to nitride semiconductor devices made by using aself-standing nonpolar or semipolar GaN substrate are anticipated(Non-patent Document 2). A nitride semiconductor is also referred to asa GaN-based semiconductor, a group III nitride-based compoundsemiconductor, a nitride-based group III-V compound semiconductor, andthe like and, in addition to including GaN (gallium nitride), includes acompound in which a part of or all of Ga in GaN is replaced with anotherelement belonging to the thirteenth group in the periodic table (B, Al,or In). Examples include AlN, InN, AlGaN, AlInN, GaInN, and AlGaInN.

Among nonpolar or semipolar GaN substrates, a (10-10) substrate, a(20-21) substrate, a (20-2-1) substrate, a (30-31) substrate, and a(30-3-1) substrate are particularly useful. The (10-10) substrate(M-plane substrate) is a nonpolar substrate, while the (20-21)substrate, the (20-2-1) substrate, the (30-31) substrate, and the(30-3-1) substrate are semipolar substrates.

A main object of the present invention is to provide a self-standingnonpolar or semipolar GaN substrate having improved size and crystalquality.

The following inventions related to a self-standing GaN substrate, acrystal, a manufacturing method of a GaN single crystal, a manufacturingmethod of a semiconductor device, and the like are provided.

(1) A self-standing GaN substrate with an angle between the normal ofthe principal surface and an m-axis of 0 degrees or more and 20 degreesor less, wherein: the size of the projected image in a c-axis directionwhen the principal surface is vertically projected on an M-plane is 10mm or more; and a low distortion section with a section length of 6 mmor more and with an a-axis length variation within the section of10.0×10⁻⁵ Å or less is observed when an a-axis length is measured on aline of intersection between the principal surface and an A-plane.(2) The self-standing GaN substrate according to (1) above, wherein thesize of the projected image in a c-axis direction is 15 mm or more, andthe low distortion section has a section length of 8 mm or more and ana-axis length variation within the section of 10.0×10⁻⁵ Å or less.(3) The self-standing GaN substrate according to (2) above, wherein thelow distortion section has a section length of 10 mm or more and ana-axis length variation within the section of 10.0×10⁻⁵ Å or less.(4) The self-standing GaN substrate according to (2) above, wherein thelow distortion section has a section length of 10 mm or more and ana-axis length variation within the section of 8.0×10⁻⁵ Å or less.(5) The self-standing GaN substrate according to any one of (1) to (4)above, wherein a c-axis length variation in the low distortion sectionis 3.0×10⁻⁴ Å or less.(6) The self-standing GaN substrate according to any one of (1) to (5)above, wherein an X-ray rocking curve full-width at half-maximum of a(300) plane in the low distortion section is less than 100 arcsec.(7) The self-standing GaN substrate according to any one of (1) to (6)above, wherein a variation width of an X-ray rocking curve full-width athalf-maximum of the (300) plane in the low distortion section is lessthan 20 arcsec.(8) A self-standing GaN substrate with an angle between the normal ofthe principal surface and an m-axis of 0 degrees or more and 20 degreesor less, having an defect increasing zone extending in a directionintersecting an A-plane on the principal surface, wherein: the size ofthe projected image in a c-axis direction when the principal surface isvertically projected on an M-plane is 10 mm or more; and when a regionexcluding a portion at a distance of 2 mm or less from a substrate endsurface, of the principal surface, is assumed to be an effective region,an a-axis length on a longest intersection line formable by theprincipal surface and the A-plane in the effective region is within arange expressed as L_(a)±5.0×10⁻⁵ Å (where L_(a) is a value equal to orlarger than 3.1885 and smaller than 3.1895) in a portion excluding thedefect increasing zone.(9) The self-standing GaN substrate according to (8) above, wherein asection with a width of less than 2 mm, where an a-axis length isoutside the said range, exists in a portion where the longestintersection line traverses the defect increasing zone.(10) The self-standing GaN substrate according to (8) or (9) above,wherein a c-axis length on the longest intersection line is within arange expressed as L_(c)±1.5×10⁻⁴ Å (where L_(c) is a value equal to orlarger than 5.1845 and smaller than 5.1865) in a portion excluding thedefect increasing zone.(11) The self-standing GaN substrate according to any one of (8) to (10)above, wherein an X-ray rocking curve full-width at half-maximum of a(300) plane on the longest intersection line is less than 100 arcsec ina portion excluding the defect increasing zone.(12) The self-standing GaN substrate according to any one of (8) to (11)above, wherein a variation width of an X-ray rocking curve full-width athalf-maximum of a (300) plane on the longest intersection line is lessthan 20 arcsec in a portion excluding the defect increasing zone.(13) A self-standing GaN substrate with an angle between the normal ofthe principal surface and an m-axis of 0 degrees or more and 20 degreesor less, wherein: the size of the projected image in a c-axis directionwhen the principal surface is vertically projected on an M-plane is 10mm or more; and when a region excluding a portion at a distance of 2 mmor less from a substrate end surface, of the principal surface, isassumed to be an effective region, an a-axis length on a longestintersection line formable by the principal surface and an A-plane inthe effective region is within a range expressed as L_(a)±5.0×10⁻⁵ Å(where L_(a) is a value equal to or larger than 3.1885 and smaller than3.1895).(14) The self-standing GaN substrate according to (13) above, wherein ac-axis length on the longest intersection line is within a rangeexpressed as L_(c)±1.5×10⁻⁴ Å (where L_(c) is a value equal to or largerthan 5.1845 and smaller than 5.1865).(15) The self-standing GaN substrate according to (13) or (14) above,wherein an X-ray rocking curve full-width at half-maximum of a (300)plane on the longest intersection line is less than 100 arcsec.(16) The self-standing GaN substrate according to any one of (13) to(15) above, wherein a variation width of an X-ray rocking curvefull-width at half-maximum of a (300) plane on the longest intersectionline is less than 20 arcsec.(17) A self-standing GaN substrate with an angle between the normal ofthe principal surface and an m-axis of 0 degrees or more and 20 degreesor less, having an defect increasing zone extending in a directionintersecting an A-plane on the principal surface, wherein: the size ofthe projected image in a c-axis direction when the principal surface isvertically projected on an M-plane is 10 mm or more; when a regionexcluding a portion at a distance of 2 mm or less from a substrate endsurface, of the principal surface, is assumed to be an effective region,an X-ray rocking curve full-width at half-maximum of a (300) plane on alongest intersection line formable by the principal surface and theA-plane in the effective region is less than 100 arcsec in a portionexcluding the defect increasing zone; and a variation width of the X-rayrocking curve full-width at half-maximum of a (300) plane on the longestintersection line is less than 20 arcsec in a portion excluding thedefect increasing zone.(18) The self-standing GaN substrate according to (17) above, wherein aspot where the X-ray rocking curve full-width at half-maximum of the(300) plane exhibits a prominently high value is comprised in a portionwhere the longest intersection line traverses the defect increasingzone.(19) A self-standing GaN substrate with an angle between the normal ofthe principal surface and an m-axis of 0 degrees or more and 20 degreesor less, wherein: the size of the projected image in a c-axis directionwhen the principal surface is vertically projected on an M-plane is 10mm or more; when a region excluding a portion at a distance of 2 mm orless from a substrate end surface, of the principal surface, is assumedto be an effective region, an X-ray rocking curve full-width athalf-maximum of a (300) plane on a longest intersection line formable bythe principal surface and an A-plane in the effective region is lessthan 100 arcsec; and a variation width of the X-ray rocking curvefull-width at half-maximum of a (300) plane on the longest intersectionline is less than 20 arcsec.(20) The self-standing GaN substrate according to any one of (1) to (19)above, wherein the size of the projected image in an a-axis directionwhen the principal surface is vertically projected on an M-plane is 30mm or more.(21) The self-standing GaN substrate according to (20) above, whereinthe size of the projected image in an a-axis direction is 40 mm or more.(22) The self-standing GaN substrate according to any one of (1) to (21)above, wherein the self-standing GaN substrate comprises a GaN crystalgrown by an HVPE method.(23) The self-standing GaN substrate according to any one of (1) to (22)above, wherein an absorption coefficient at a wavelength of 450 nm is 2cm⁻¹ or less.(24) The self-standing GaN substrate according to any one of (1) to (7),(13) to (16), and (19) to (21) above, wherein the self-standing GaNsubstrate contains fluorine.(25) The self-standing GaN substrate according to any one of (1) to (24)above, wherein alkali metal concentration is lower than 1×10¹⁵ cm⁻³.(26) The self-standing GaN substrate according to any one of (1) to (25)above, wherein the self-standing GaN substrate contains a stackingfault.(27) A crystal comprising GaN, processing of which enables fabricationof the free-standing GaN substrate according to any one of (1) to (26)above.(28) A manufacturing method of a GaN single crystal, including preparingthe self-standing GaN substrate according to any one of (1) to (26)above and epitaxially growing GaN on the self-standing GaN substrate.(29) A manufacturing method of a GaN single crystal, including growing afirst GaN crystal using the self-standing GaN substrate according to anyone of (1) to (26) above as a seed, and subsequently growing a secondGaN crystal using a part of or all of the first GaN crystal as a seed.(30) The manufacturing method according to (28) or (29), wherein themanufacturing method is a manufacturing method of a bulk GaN singlecrystal.(31) A manufacturing method of a semiconductor device, includingpreparing the self-standing GaN substrate according to any one of (1) to(26) above and forming a device structure by epitaxially growing one ormore types of nitride semiconductors on the self-standing GaN substrate.(32) A manufacturing method of a GaN layer-bonded substrate, includingthe steps of: implanting ions in a vicinity of the principal surface ofthe self-standing GaN substrate according to any one of (1) to (26)above; bonding the principal surface side of the self-standing GaNsubstrate to a hetero-composition substrate; and forming a GaN layerbonded to the hetero-composition substrate by separating theself-standing GaN substrate at the ion-implanted region as a boundary.(33) A GaN layer-bonded substrate with a structure in which a GaN layerseparated from the self-standing GaN substrate according to any one of(1) to (26) above is bonded to a hetero-composition substrate.

In addition, the following inventions related to a self-standing GaNsubstrate, a crystal, a method of producing a self-standing GaNsubstrate, a manufacturing method of a GaN single crystal, amanufacturing method of a semiconductor device, and the like areprovided.

(1a) A self-standing GaN substrate with an angle between the normal ofthe principal surface and an m-axis of 0 degrees or more and 20 degreesor less, wherein the size of the projected image in a c-axis directionwhen the principal surface is vertically projected on an M-plane is 10mm or more, and an anomalous transmission image is obtained bytransmission X-ray topography.(2a) The self-standing GaN substrate according to (1a) above, whereinthe size of the projected image is a 10 mm square or more.(3a) The self-standing GaN substrate according to (1a) or (2a) above,wherein the size of the projected image in a c-axis direction is 15 mmor more.(4a) The self-standing GaN substrate according to any one of (1a) to(3a) above, wherein the size of the projected image in an a-axisdirection is 30 mm or more.(5a) The self-standing GaN substrate according to any one of (1a) to(4a) above, wherein the self-standing GaN substrate comprises a GaNcrystal grown by an HVPE method.(6a) The self-standing GaN substrate according to any one of (1a) to(5a) above, wherein an absorption coefficient at a wavelength of 450 nmis 2 cm⁻¹ or less.(7a) The self-standing GaN substrate according to any one of (1a) to(4a) above, wherein the self-standing GaN substrate contains fluorine.(8a) The self-standing GaN substrate according to any one of (1a) to(7a) above, wherein alkali metal concentration is lower than 1×10¹⁵cm⁻³.(9a) The self-standing GaN substrate according to any one of (1a) to(8a) above, wherein the self-standing GaN substrate contains a stackingfault.(10a) A crystal which comprises GaN and which is processed in order tofabricate the self-standing GaN substrate according to any one of (1a)to (9a) above.(11a) A method of producing the self-standing GaN substrate according toany one of (1a) to (9a) above, the method having an inspection stepincluding transmission X-ray topography using anomalous transmission asa test item, wherein a product in which an unacceptable defect is foundin the inspection step is deemed a defective product.(12a) A manufacturing method of a GaN single crystal, includingpreparing the self-standing GaN substrate according to any one of (1a)to (9a) above and epitaxially growing GaN on the self-standing GaNsubstrate.(13a) A manufacturing method of a GaN single crystal, including growinga first GaN crystal using the self-standing GaN substrate according toany one of (1a) to (9a) above as a seed, and subsequently growing asecond GaN crystal using a part of or all of the first GaN crystal as aseed.(14a) The manufacturing method according to (12a) or (13a), wherein themanufacturing method is a manufacturing method of a bulk GaN singlecrystal.(15a) A manufacturing method of a semiconductor device, includingpreparing the self-standing GaN substrate according to any one of (1a)to (9a) above and forming a device structure by epitaxially growing oneor more types of nitride semiconductors on the self-standing GaNsubstrate.(16a) A manufacturing method of a GaN layer-bonded substrate, includingthe steps of: implanting ions in a vicinity of the principal surface ofthe self-standing GaN substrate according to any one of (1a) to (9a)above; bonding the principal surface side of the self-standing GaNsubstrate to a hetero-composition substrate; and forming a GaN layerbonded to the hetero-composition substrate by separating theself-standing GaN substrate at the ion-implanted region as a boundary.(17a) A GaN layer-bonded substrate with a structure in which a GaN layerseparated from the self-standing GaN substrate according to any one of(1a) to (9a) above is bonded to a hetero-composition substrate.

Furthermore, the following inventions related to a self-standing GaNsubstrate, a manufacturing method of a GaN single crystal, amanufacturing method of a semiconductor device, and the like areprovided.

(1b) A self-standing GaN substrate with an angle between the normal ofthe principal surface and an m-axis of 0 degrees or more and 20 degreesor less, wherein: the size of the projected image in a c-axis directionwhen the principal surface is vertically projected on an M-plane is 10mm or more; and when a region excluding a portion at a distance of 2 mmor less from a substrate end surface, of the principal surface, isassumed to be an effective region, a dislocation density obtained bydividing a total number of dislocations existing in the effective regionby an area of the effective region is less than 4×10⁵ cm⁻².(2b) The self-standing GaN substrate according to (1b) above, whereinthe size of the projected image is a 10 mm square or more.(3b) The self-standing GaN substrate according to (1b) or (2b) above,wherein the size of the projected image in a c-axis direction is 15 mmor more.(4b) The self-standing GaN substrate according to any one of (1b) to(3b) above, wherein the size of the projected image in an a-axisdirection is 30 mm or more.(5b) The self-standing GaN substrate according to any one of (1b) to(4b) above, wherein the self-standing GaN substrate contains fluorine.(6b) The self-standing GaN substrate according to any one of (1b) to(5b) above, wherein alkali metal concentration is lower than 1×10¹⁵cm⁻³.(7b) The self-standing GaN substrate according to any one of (1b) to(6b) above, wherein the self-standing GaN substrate contains a stackingfault.(8b) A manufacturing method of a GaN single crystal, including preparingthe self-standing GaN substrate according to any one of (1b) to (7b)above and epitaxially growing GaN on the self-standing GaN substrate.(9b) A manufacturing method of a GaN single crystal, including growing afirst GaN crystal using the self-standing GaN substrate according to anyone of (1b) to (7b) above as a seed, and subsequently growing a secondGaN crystal using a part of or all of the first GaN crystal as a seed.(10b) The manufacturing method according to (8b) or (9b), wherein themanufacturing method is a manufacturing method of a bulk GaN singlecrystal.(11b) A manufacturing method of a semiconductor device, includingpreparing the self-standing GaN substrate according to any one of (1b)to (7b) above and forming a device structure by epitaxially growing oneor more types of nitride semiconductors on the self-standing GaNsubstrate.(12b) A manufacturing method of a GaN layer-bonded substrate, includingthe steps of: implanting ions in a vicinity of the principal surface ofthe self-standing GaN substrate according to any one of (1b) to (7b)above; bonding the principal surface side of the self-standing GaNsubstrate to a hetero-composition substrate; and forming a GaN layerbonded to the hetero-composition substrate by separating theself-standing GaN substrate at the ion-implanted region as a boundary.(13b) A GaN layer-bonded substrate with a structure in which a GaN layerseparated from the self-standing GaN substrate according to any one of(1b) to (7b) above is bonded to a hetero-composition substrate.

Moreover, the following inventions related to a self-standing GaNsubstrate, a manufacturing method of a GaN single crystal, amanufacturing method of a semiconductor device, and the like areprovided.

(1c) A self-standing GaN substrate with an angle between the normal ofthe principal surface and an m-axis of 0 degrees or more and 20 degreesor less, wherein: the size of the projected image in a c-axis directionwhen the principal surface is vertically projected on an M-plane is 10mm or more; and when a region excluding a portion at a distance of 2 mmor less from a substrate end surface, of the principal surface, isassumed to be an effective region, a stacking fault density obtained bydividing a total length of stacking faults existing in the effectiveregion by an area of the effective region is less than 15 cm⁻¹.(2c) The self-standing GaN substrate according to (1c) above, whereinthe size of the projected image is a 10 mm square or more.(3c) The self-standing GaN substrate according to (1c) or (2c) above,wherein the size of the projected image in a c-axis direction is 15 mmor more.(4c) The self-standing GaN substrate according to any one of (1c) to(3c) above, wherein the size of the projected image in an a-axisdirection is 30 mm or more.(5c) The self-standing GaN substrate according to any one of (1c) to(4c) above, wherein the self-standing GaN substrate contains fluorine.(6c) The self-standing GaN substrate according to any one of (1c) to(5c) above, wherein alkali metal concentration is lower than 1×10¹⁵cm⁻³.(7c) The self-standing GaN substrate according to any one of (1c) to(6c) above, wherein the self-standing GaN substrate contains a stackingfault.(8c) A manufacturing method of a GaN single crystal, including preparingthe self-standing GaN substrate according to any one of (1c) to (7c)above and epitaxially growing GaN on the self-standing GaN substrate.(9c) A manufacturing method of a GaN single crystal, including growing afirst GaN crystal using the self-standing GaN substrate according to anyone of (1c) to (7c) above as a seed, and subsequently growing a secondGaN crystal using a part of or all of the first GaN crystal as a seed.(10c) The manufacturing method according to (8c) or (9c), wherein themanufacturing method is a manufacturing method of a bulk GaN singlecrystal.(11c) A manufacturing method of a semiconductor device, includingpreparing the self-standing GaN substrate according to any one of (1c)to (7c) above and forming a device structure by epitaxially growing oneor more types of nitride semiconductors on the self-standing GaNsubstrate.(12c) A manufacturing method of a GaN layer-bonded substrate, includingthe steps of: implanting ions in a vicinity of the principal surface ofthe self-standing GaN substrate according to any one of (1c) to (7c)above; bonding the principal surface side of the self-standing GaNsubstrate to a hetero-composition substrate; and forming a GaN layerbonded to the hetero-composition substrate by separating theself-standing GaN substrate at the ion-implanted region as a boundary.(13c) A GaN layer-bonded substrate with a structure in which a GaN layerseparated from the self-standing GaN substrate according to any one of(1c) to (7c) above is bonded to a hetero-composition substrate.

The following inventions related to a GaN crystal and a manufacturingmethod of a self-standing GaN substrate are provided.

(1d) A manufacturing method of a GaN crystal, including: a seedpreparation step of preparing a seed including a first GaN crystal grownusing an ammonothermal method; and a crystal growth step of growing asecond GaN crystal having better heat resistance than the first GaNcrystal on the seed using an ammonothermal method.(2d) The manufacturing method according to (1d) above, wherein in thecrystal growth step, a stacking fault is formed in the second GaNcrystal.(3d) The manufacturing method according to (1d) or (2d) above, whereinin the crystal growth step, an acidic mineralizer containing fluorine isused.(4d) A manufacturing method of a GaN crystal, including: a first step ofgrowing a first GaN crystal using an ammonothermal method; a second stepof fabricating a seed comprising the first GaN crystal; and a third stepof growing a second GaN crystal having better heat resistance than thefirst GaN crystal on the seed using an ammonothermal method.(5d) The manufacturing method according to (4d) above, wherein in thefirst step, the first GaN crystal is grown on a nitrogen polaritysurface of GaN.(6d) The manufacturing method according to (5d) above, wherein regrowthis performed in the first step.(7d) The manufacturing method according to any one of (4d) to (6d)above, wherein in the third step, a stacking fault is formed in thesecond GaN crystal.(8d) The manufacturing method according to any one of (4d) to (7d)above, wherein in the third step, an acidic mineralizer containingfluorine is used.(9d) The manufacturing method according to any one of (4d) to (8d)above, wherein the size in a c-axis direction of the seed fabricated inthe second step is 10 mm or more.(10d) The manufacturing method according to any one of (1d) to (9d)above, wherein the seed is an M-plane GaN substrate.(11d) A manufacturing method of a self-standing GaN substrate, includingthe steps of: manufacturing a GaN crystal using the manufacturing methodaccording to any one of (1d) to (10d) above; and cutting out aself-standing substrate from the GaN crystal.(12d) The manufacturing method according to (11d) above, wherein anangle between a normal of a principal surface of the self-standingsubstrate and an m-axis is 0 degrees or more and 20 degrees or less.(13d) The manufacturing method according to (12d) above, wherein thesize of the projected image in a c-axis direction when the principalsurface of the self-standing substrate is vertically projected on anM-plane is 10 mm or more.

According to preferred embodiments of the present invention, a nonpolaror semipolar GaN substrate with improved size and crystal quality isprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a GaN crystal whose sizes in ac-axis direction, an a-axis direction, and an m-axis direction arerespectively 10 mm, 20 mm, and 3.4 mm.

FIG. 2 is a graph showing a result of measurements of an a-axis lengthon a principal surface of a self-standing M-plane GaN substrateaccording to an embodiment taken every 250 μm on an intersection linebetween the principal surface and an A-plane.

FIG. 3 is a diagram for explaining a longest intersection line betweenthe principal surface and an A-plane in a disk-shaped M-plane GaNsubstrate.

FIG. 4 is a perspective view of a disk-shaped M-plane GaN substratehaving an defect increasing zone on a principal surface thereof.

FIG. 5 is a conceptual diagram of a vapor phase growth apparatus used inan HVPE method.

FIG. 6 is a perspective view of a C-plane GaN substrate having a growthmask with a stripe pattern formed on a nitrogen polarity surface.

FIG. 7 is a conceptual diagram of a high-pressure growth apparatus usedin an ammonothermal method.

FIG. 8 is a perspective view showing how a GaN crystal grows on aC-plane GaN substrate having a growth mask with a stripe pattern formedon a nitrogen polarity surface.

FIG. 9 is a perspective view showing a structure formed by a GaN crystalgrowing ammonothermally on a C-plane GaN substrate having a growth maskwith a stripe pattern formed on a nitrogen polarity surface.

FIG. 10 is a diagram explaining that an appearance of a stable planethat is inclined with respect to the growth direction on the surfacecauses a growth rate of a GaN crystal to slow down, wherein FIG. 10(a)illustrates a seed substrate before the GaN crystal is grown, FIG. 10(b)illustrates a state where a stable plane has appeared on the surface ofthe GaN crystal growing on the seed substrate, and FIG. 10(c)illustrates a state where the entire surface of the GaN crystal growingon the seed substrate is occupied by the stable plane.

FIG. 11 (a) is a perspective view illustrating a self-standing M-planeGaN substrate whose principal surface is rectangular and two sides amongfour sides constituting the rectangle are parallel to an a-axis of a GaNcrystal and the other two sides are parallel to a c-axis of the GaNcrystal, and FIG. 11(b) is a side view of the self-standing M-plane GaNsubstrate as observed from the side of an A end surface.

FIG. 12(a) is a plan view of an aggregated seed formed by arranging fiveM-plane GaN substrates side by side so that principal surfaces thereofface upward, and FIG. 12(b) is a sectional view showing a state where aGaN crystal grown by an HVPE method has covered the aggregated seedshown in FIG. 12 (a).

FIG. 13 shows an example of a profile of a susceptor temperatureadoptable in a two-step growth method.

FIG. 14 is a diagram showing an arrangement of an X-ray source, a testpiece, and a detector in transmission X-ray topography by Lang's method.

FIG. 15 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 16 is a graph showing a result of measurements of a c-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 17 is a graph showing a result of measurements of an X-ray rockingcurve full-width at half-maximum (XRC-FWHM) of a (300) plane on aprincipal surface of an M-plane GaN substrate taken every 250 μm on astraight line parallel to a c-axis.

FIG. 18 shows a reflection X-ray topographic image of an M-plane GaNsubstrate obtained using (203) diffraction.

FIG. 19 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 20 is a graph showing a result of measurements of a c-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 21 shows a reflection X-ray topographic image of an M-plane GaNsubstrate obtained using (203) diffraction.

FIG. 22 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 23 is a graph showing a result of measurements of a c-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 24 (a) is an external view photograph of an M-plane GaN substrate,and FIG. 24 (b) is a transmission X-ray topographic image of a part ofthe M-plane GaN substrate obtained using (002) diffraction.

FIG. 25 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 26 is a graph showing a result of measurements of a c-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 27(a) is an external view photograph of a test piece cut out froman M-plane GaN substrate, FIG. 27(b) is a transmission X-ray topographicimage of the test piece obtained using (002) diffraction, FIG. 27(c) isa transmission X-ray topographic image of the test piece obtained using(110) diffraction, and FIG. 27 (d) is a reflection X-ray topographicimage of the test piece obtained using (203) diffraction.

FIG. 28(a) is an external view photograph of an M-plane GaN substrate,and FIG. 28(b) is a transmission X-ray topographic image of the M-planeGaN substrate obtained using (002) diffraction.

FIG. 29 is a plan view showing five straight lines arranged at 5 mmintervals in an a-axis direction on a principal surface of an M-planeGaN substrate.

FIG. 30 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 31 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 32 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 33 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 34 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 35 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to an a-axis.

FIG. 36 is a graph showing a result of measurements of an a-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 37 is a graph showing a result of measurements of a c-axis lengthon a principal surface of an M-plane GaN substrate taken every 250 μm ona straight line parallel to a c-axis.

FIG. 38 is a graph showing a result of measurements of an a-axis lengthon a principal surface of a GaN (30-3-1) substrate taken every 250 μm onan intersection line between the principal surface and an A-plane.

FIG. 39 is a graph showing a result of measurements of a c-axis lengthon a principal surface of a GaN (30-3-1) substrate taken every 250 μm onan intersection line between the principal surface and an A-plane.

FIG. 40 is a graph showing a result of measurements of an a-axis lengthon a principal surface of a GaN (20-21) substrate taken every 250 μm onan intersection line between the principal surface and an A-plane.

FIG. 41 is a graph showing a result of measurements of a c-axis lengthon a principal surface of a GaN (20-21) substrate taken every 250 μm onan intersection line between the principal surface and an A-plane.

BEST MODE FOR CARRYING OUT THE INVENTION

A GaN crystal has a wurtzite type crystal structure belonging to thehexagonal crystal system.

In a GaN crystal, a crystal axis parallel to [0001] is called a c-axis,a crystal axis parallel to [10-10] is called an m-axis, and a crystalaxis parallel to [11-20] is called an a-axis. In addition, a crystalplane perpendicular to the c-axis is called a C-plane, a crystal planeperpendicular to the m-axis is called an M-plane, and a crystal planeperpendicular to the a-axis is called an A-plane.

In the present specification, unless otherwise noted, references made toa crystal axis, a crystal plane, a crystal orientation, and the like areassumed to mean a crystal axis, a crystal plane, a crystal orientation,and the like of a GaN crystal.

In the present specification, references may be made to an intersectionline between a principal surface of a GaN substrate and an A-plane. TheA-plane in this case is assumed to mean an A-plane perpendicular to theM-plane that is parallel to or closest to parallel to the principalsurface.

A name or Miller indices of a crystal plane that is attached to a nameof a GaN substrate is that of a low index plane that is parallel to orclosest to parallel to a front surface of the substrate. The frontsurface refers to a surface intended to be used for forming asemiconductor device or epitaxially growing a crystal among the twoprincipal surfaces of the substrate. The principal surface that is notthe front surface is referred to as a back surface.

For example, a GaN substrate in which a low index plane that is parallelto or closest to parallel to a front surface thereof is an M-plane or,in other words, (10-10) is referred to as an M-plane substrate or a(10-10) substrate. Normally, a crystal plane for which absolute valuesof integers h, k, m, and l of Miller indices (hkml) are equal to orsmaller than 3 is assumed to be a low index plane.

Hereinafter, the present invention will be described in detail in linewith embodiments thereof.

1. Self-Standing GaN Substrate

A self-standing GaN substrate refers to a single crystal substrateconsisting only of GaN crystal. In the present specification, aself-standing GaN substrate may also be simply referred to as a GaNsubstrate.

A self-standing GaN substrate according to the present invention needonly be thick enough to allow handling as a self-standing substrate.Although the thickness varies depending on the size of the principalsurface, the thickness can be empirically determined. In the case of a 2inch-diameter disk-shaped substrate, a minimum thickness that enablesthe substrate to be handled as a self-standing substrate is normally 150to 200 μm, a favorable thickness is 250 μm or more, and a furtherfavorable thickness is 280 μm or more.

While the thickness of a self-standing GaN substrate according to thepresent invention does not particularly have an upper limit, normally,the thickness is 1.5 mm or less. In the case of a 2 inch-diameterdisk-shaped substrate, a thickness thereof is normally 400 μm or lessand favorably 350 μm or less. However, depending on use a thickersubstrate may be more favorable.

In a self-standing GaN substrate according to the present invention, anangle between the normal of at least one principal surface and an m-axisis 0 degrees or more and 20 degrees or less. In other words, at leastone principal surface of a GaN substrate according to the presentinvention is parallel to a crystal plane whose angle of inclination froman M-plane is 0 degrees or more and 20 degrees or less.

For example, [10-10], [20-21], [20-2-1], [30-31], and [30-3-1] form anangle with an m-axis that is 0 degrees or more and 20 degrees or less.Therefore, a (10-10) substrate, a (20-21) substrate, a (20-2-1)substrate, a (30-31) substrate, and a (30-3-1) substrate are included ina GaN substrate in which an angle between the normal of the principalsurface and an m-axis is 0 degrees or more and 20 degrees or less.

In a GaN substrate according to the present invention, the higher theparallelism between two principal surfaces, the more favorable. When thetwo principal surfaces are parallel to each other, an angle between thenormal and an m-axis is 0 degrees or more and 20 degrees or less at eachprincipal surface.

In a GaN substrate according to the present invention, the size of theprojected image in a c-axis direction when the principal surface isvertically projected on a M-plane is 10 mm or more. The size in a c-axisdirection is favorably 15 mm or more, more favorably 20 mm or more, morefavorably 25 mm or more, more favorably 30 mm or more, more favorably 35mm or more, more favorably 40 mm or more, more favorably 45 mm or more,and more favorably 50 mm or more. In addition, the size of the projectedimage in an a-axis direction is normally 10 mm or more, favorably 20 mmor more, more favorably 30 mm or more, more favorably 40 mm or more, andmore favorably 50 mm or more.

For the purpose of illustration, FIG. 1 shows a GaN crystal whose sizesin an a-axis direction, an c-axis direction, and an m-axis direction arerespectively 20 mm, 10 mm, and 3.4 mm. A person skilled in the art wouldunderstand that, by slicing this GaN crystal, a substrate can beobtained in which an angle of inclination of the principal surface withrespect to an M-plane is 10 degrees or less in an a-axis direction andwithin ±20 degrees in a c-axis direction and in which the size of theprojected image when the principal surface is vertically projected onthe M-plane is 20 mm in an a-axis direction and 10 mm in a c-axisdirection. The smaller the inclination angle of the principal surfacewith respect to the M-plane, a larger number of the substrates can becut out.

2. Preferred Embodiments

Various embodiments of the present invention will be described below.

2.1. First Embodiment 2.1.1. Self-Standing GaN Substrate

With a self-standing GaN substrate according to the first embodiment,when an a-axis length is measured on an intersection line between theprincipal surface and an A-plane, a low distortion section with anextremely small variation in the a-axis length is observed.

The a-axis length variation in the low distortion section is favorably10.0×10⁻⁵ Å or less, more favorably 8.0×10⁻⁵ Å or less, and mostfavorably 6.0×10⁻⁵ Å or less. A section length of the low distortionsection is favorably 6 mm or more, more favorably 8 mm or more, and mostfavorably 10 mm or more.

The a-axis length can be measured using a reflection mode X-raydiffractometer. Measurement intervals are favorably 250 μm or less. Ana-axis length is calculated based on a (300) lattice spacing obtainingby performing a 2θ-ω scan of a (300) plane at each measurement point.

FIG. 2 shows a result of measurements of an a-axis length on a principalsurface of a 50 mm (a-axis direction)×25 mm (c-axis direction)×346 μm(m-axis direction) self-standing M-plane GaN substrate according to thefirst embodiment as taken every 250 μm on a straight line parallel to ac-axis. In the case of an M-plane substrate, an intersection linebetween the principal surface and an A-plane is parallel to a c-axis.

In FIG. 2, an a-axis length at each measurement point is plotted on acoordinate plane in which a horizontal axis thereof represents aposition in a c-axis direction and a vertical axis thereof represents ana-axis length. The graph shown in FIG. 2 has three low distortionsections, namely, a section A, a section B, and a section C. Among therespective low distortion sections, the section A has an a-axis lengthvariation width of 3.8×10⁻⁵ Å, the section B has an a-axis lengthvariation width of 3.5×1.0⁻⁵ Å, and the section C has an a-axis lengthvariation width of 3.8×10⁻⁵ Å. The section A has a section length ofapproximately 7 mm and the sections B and C respectively have sectionlengths of approximately 8 mm.

When a low distortion section is observed in a measurement of a-axislengths on one straight line, the GaN substrate may be described asincluding a low distortion region extending also in a direction thatintersects with the straight line.

Due to having a low distortion region, a GaN substrate according to thefirst embodiment has an advantage in that deformation occurring when theGaN substrate is heated inside a vapor phase growth furnace is small.Vapor phase epitaxial growth of a nitride semiconductor is normallyperformed while heating a substrate at a high temperature of 800° C. orhigher.

In the low distortion section described above, furthermore, a variationin a c-axis length is favorably 3.0×10⁻⁴ Å or less and more favorablyless than 2.0×10⁻⁴ Å.

In the low distortion section described above, furthermore, an X-rayrocking curve full-width at half-maximum (XRC-FWHM) of a (300) plane isfavorably less than 100 arcsec and more favorably less than 90 arcsec.Generally, XRC-FWHM is smaller when dislocation density of a crystal islow.

In the low distortion section described above, furthermore, a variationin the XRC-FWHM of a (300) plane is favorably less than 20 arcsec andmore favorably less than 10 arcsec.

When measuring an a-axis length, a c-axis length, and/or an XRC-FWHM onintersection lines between a principal surface of a substrate and anA-plane, favorably, a longest one among the intersection lines may beselected and the measurement may be performed thereon. A longest oneamong the intersection lines between a principal surface of a substrateand an A-plane means a longest intersection line formable by theprincipal surface and an A-plane. For example, in the case of adisk-shaped M-plane GaN substrate, as shown in FIG. 3, a longestintersection line between the principal surface and an A-plane is astraight line (a dashed line in FIG. 3) parallel to a c-axis and passingthrough the center of the principal surface (center of a circle).

When evaluating a crystal quality of a GaN substrate based on a resultof an X-ray measurement, a portion at a distance of 2 mm or less from asubstrate end surface may be excluded from consideration. This isbecause, in an outer peripheral portion of a substrate, defectsattributable to mechanical processing of a crystal tend to remain andthere are cases when a measurement value does not accurately reflectcrystal quality.

Considering above, a region excluding portions at a distance of 2 mm orless from a substrate end surface, of a principal surface of asubstrate, may be assumed to be an effective region, an a-axis lengthmay be measured on a longest intersection line formable by the principalsurface and an A-plane in the effective region, and crystal quality ofthe substrate may be evaluated based on a result of the measurement.

With a self-standing GaN substrate according to a preferred example, ona longest intersection line formable by the principal surface and anA-plane in the effective region, an a-axis length is within a rangeexpressed as L_(a)±5.0×10⁻⁵ Å (where L_(a) is a value equal to or largerthan 3.1885 and smaller than 3.1895).

The a-axis length being within a range expressed as L_(a)±5.0×10⁻⁵ Åmeans that a variation width of the a-axis length on the longestintersection line is 10.0×10⁻⁵ Å or less. L_(a) (A) denotes a centervalue of the a-axis length on the longest intersection line and may be3.1885 or larger and smaller than 3.1895.

In a more favorable example, the a-axis length on the longestintersection line is within a range expressed as L_(a)±4.0×10⁻⁵ Å andfurthermore within a range expressed as L_(a)±3.0×10⁻⁵ Å.

On the longest intersection line formable by a principal surface of asubstrate and an A-plane in the effective region, furthermore, a c-axislength is favorably within a range expressed as L_(c)±1.5×10⁻⁴ Å (whereL_(c) is a value equal to or larger than 5.1845 and smaller than5.1865). The c-axis length being within a range expressed asL_(c)±1×10⁻⁴ Å means that a variation width of the c-axis length on thelongest intersection line is 3.0±10⁻⁴ Å or less. L_(c) (Å) denotes acenter value of the c-axis length on the longest intersection line andmay be 5.1845 or larger and smaller than 5.1865.

In a more favorable example, the c-axis length on the longestintersection line is within a range expressed as L_(c)±1.0×10⁻⁵ Å.

Otherwise, on the longest intersection line formable by a principalsurface of a substrate and an A-plane in the effective region, anXRC-FWHM of a (300) plane is favorably less than 100 arcsec and morefavorably less than 90 arcsec, and a variation width in the XRC-FWHM ofa (300) plane is favorably less than 20 arcsec and more favorably lessthan 10 arcsec.

A self-standing GaN substrate according to the first embodiment may havean defect increasing zone extending in a direction intersecting anA-plane on the principal surface.

An defect increasing zone refers to a belt-like region in which densityof crystal defects such as dislocations and stacking faults is locallyhigh and is observed when a GaN crystal is grown by a method using anaggregated seed to be described later and a GaN substrate is fabricatedfrom the GaN crystal. Specifically, in a GaN crystal grown on anaggregated seed, a portion with high defect density grown above aboundary between seed substrates constituting the aggregated seedappears as an defect increasing zone on a surface of a GaN substrateobtained by processing the GaN crystal.

Typically, when dislocation density and stacking fault density measuredin an arbitrarily-selected 300 μm×400 μm area are respectively 1×10⁴ to1×10⁶ cm⁻² and 0 to 5×10² cm⁻¹ outside the defect increasing zone, thesedefect densities inside the defect increasing zone are respectively1×10⁷ to 1×10⁹ cm⁻² and 1×10³ to 1×10⁵ cm⁻¹.

FIG. 4 is a perspective view of a disk-shaped M-plane GaN substratehaving an defect increasing zone on a principal surface thereof. Thedefect increasing zone extends along an a-axis direction. A dotted linedepicts an outer edge of the effective region of the substrate and adashed line depicts a longest intersection line formable by a principalsurface of the substrate and an A-plane in the effective region.

With a GaN substrate having an defect increasing zone on a principalsurface thereof, an a-axis length on a longest intersection lineformable by the principal surface and a A-plane in the effective regionis within a range expressed as favorably L_(a)±5.0×10⁻⁵ Å, morefavorably L_(a)±4.0×10⁻⁵ Å, and most favorably L_(a)±3.0×10⁻⁵ Å in aportion excluding the defect increasing zone. In this case, L_(a) may bea value equal to or larger than 3.1885 and smaller than 3.1895.

While a section in which an a-axis length falls outside the favorableranges described above may be formed in a portion where the longestintersection line traverses the defect increasing zone, a length of thissection is favorably less than 2 mm and more favorably less than 1.5 mm.

In this section, a c-axis length may also fall outside the preferredranges described above, and, in addition, an XRC-FWHM of a (300) planemay also increase locally.

While a self-standing GaN substrate according to the first embodiment isnot limited by a manufacturing method of a GaN crystal used as amaterial, in a preferred example, the self-standing GaN substratecomprises a GaN crystal grown by an HVPE method.

With an HVPE method, a GaN crystal can be grown in which concentrationsof undesirable impurities are kept lower than in a flux method or anammonothermal method.

For example, with a flux method, it is difficult to obtain a GaN crystalwith a combined alkali metal concentration of lithium (Li), sodium (Na),and potassium (K) that is lower than 1×10¹⁵ cm⁻³ (Japanese PatentApplication Laid-open No. 2009-18961). The same can be said of anammonothermal method using an alkali metal as a mineralizer (JapanesePatent Application Laid-open No. 2011-523931). In contrast, a GaNcrystal grown by an HVPE method normally has an alkali metalconcentration of lower than 1×10¹⁵ cm⁻³.

A low concentration of alkali metals in a substrate is advantageous interms of improving reliability of a semiconductor device to be formed onthe substrate.

In addition, since an HVPE method enables dopant concentration to bereadily controlled, a GaN crystal can be obtained in which carrierconcentration and conductivity are controlled with higher precision ascompared with a flux method and an ammonothermal method.

Furthermore, a GaN crystal grown by an HVPE method has high transparencyin a visible wavelength range and is suitable as a material of a GaNsubstrate for a light-emitting device. For example, at 450 nm that is anemission wavelength of an excitation blue LED used in a white LED, whilean absorption coefficient of a GaN crystal grown by an ammonothermalmethod is 4 to 20 cm⁻¹, an absorption coefficient of a GaN crystal grownby an HVPE method is 2 cm⁻¹ or less (T. Hashimoto, et al., Sensors andMaterials, Vol. 25, No. 3 (2013) 155-164).

From the perspective of productivity, an HVPE method has an advantage ofenabling a GaN crystal to grow at a significantly higher rate than aflux method and an ammonothermal method.

2.1.2. Manufacturing Method of Self-Standing GaN Substrate First Method

A self-standing GaN substrate according to the first embodiment may bemanufactured according to, but not limited to, the following procedure.

(i) Growing a primary GaN crystal by an HVPE method or the like andfabricating a C-plane substrate (primary substrate) from the primary GaNcrystal.(ii) Using the primary substrate as a seed, growing a secondary GaNcrystal by an ammonothermal method and fabricating an M-plane substrate(secondary substrate) from the secondary GaN crystal.(iii) Using the secondary substrate as a seed, growing a tertiary GaNcrystal by an ammonothermal method and fabricating an M-plane substrate(tertiary substrate) from the tertiary GaN crystal.(iv) Using the tertiary substrate as a seed, growing a quaternary GaNcrystal by an HVPE method and fabricating the self-standing GaNsubstrate according to the first embodiment by processing the quaternaryGaN crystal.

There are three important points when growing the secondary GaN crystalin step (ii) above.

Firstly, the secondary GaN crystal is grown on a nitrogen polaritysurface of the primary substrate.

Secondly, before growing the secondary GaN crystal, a growth mask of aspecific pattern (to be described later) is formed on the nitrogenpolarity surface of the primary substrate.

Thirdly, an acidic mineralizer is used.

By executing these three points, the secondary GaN crystal with smalldistortion in a crystal structure thereof can be grown at a high rateuntil a large size is attained. As a result, an area of the secondarysubstrate can be increased and, consequently, an area of the tertiarysubstrate can be increased.

It is also important that step (iii) is provided between steps (ii) and(iv). Step (iii) is necessary because the secondary substrate fabricatedin step (ii) cannot withstand temperature conditions that apply whengrowing a GaN crystal by an HVPE method. Due to a large number of voidscontained in the secondary substrate, the secondary substrate breaks ordeteriorates when subjected to high temperatures near 1000° C.

In contrast, since a GaN crystal can be grown at 650° C. or lower in anammonothermal method, the secondary substrate can be used as a seed.

The tertiary substrate fabricated from the tertiary GaN crystal grown onthe secondary substrate in step (iii) is similar to the secondarysubstrate in that distortion of a crystal structure thereof is small,and since the tertiary substrate is significantly superior to thesecondary substrate with respect to heat resistance, the tertiarysubstrate can be favorably used as a seed when growing a GaN crystal byan HVPE method.

There are two important points with respect to step (iv). Firstly, allof or approximately all of a carrier gas used when growing a GaN crystalby an HVPE method is constituted by nitrogen gas. Secondly, an uppersurface of the seed is widened as much as possible.

In prior art, use of carrier gas with a high nitrogen gas content tendedto be avoided. This is because increasing a proportion of nitrogen gasin carrier gas increases the tendency to produce polycrystalline GaN.Various problems arise under conditions where polycrystalline GaN tendsto be produced including a problem of consumption of raw material gasdue to growth of polycrystalline GaN on a side surface of a seed, aproblem in that a seed tends to adhere to a susceptor, and a problem ofdeterioration of quartz members such as a growth vessel due todeposition of polycrystalline GaN on a surface thereof.

In addition, when nitrogen gas constitutes almost all of the carriergas, stress generated by deposition of polycrystalline GaN on a sidesurface of a seed causes distortion of a GaN crystal epitaxially grownon an upper surface of the seed.

Therefore, the present inventors tried to use an aggregated seed inwhich a plurality of tertiary substrates are arranged so that respectiveend surfaces thereof come into close contact with each other and alsotried to widen a principal surface of each tertiary substrate.

As a result, an influence of polycrystalline GaN deposited on a sidesurface of the aggregated seed declined and an effect of carrier gas onthe quality of a GaN crystal epitaxially grown on an upper surface ofthe aggregated seed became apparent. In addition, it was demonstratedthat, on the tertiary substrate with high crystallinity, distortion ofan epitaxially grown GaN crystal is significantly reduced when all of orapproximately all of the carrier gas is constituted by nitrogen gas.

Details of each step will be described below.

(i) Growth of Primary GaN Crystal and Fabrication of Primary Substrate(C-Plane GaN Substrate)

The primary GaN crystal to be used as a material of the primarysubstrate may be grown by any method. A case where an HVPE method isused will now be described as an example.

When growing the primary GaN crystal using an HVPE method, a C-plane GaNtemplate can be used as a seed.

A C-plane GaN template refers to a composite substrate which uses asingle crystal substrate with a different chemical composition from GaNas a base member and which comprises a single crystal GaN layer grownthereon along c-axis by an MOCVD method or the like. The surface of thesingle crystal GaN layer is a gallium polarity face.

The base member of the C-plane GaN template is a sapphire substrate, aGaAs substrate, a SiC substrate, a Si substrate, a Ga₂O₃ substrate, anAlN substrate, or the like and a diameter thereof is normally 1 to 6inches. A thickness of the single crystal GaN layer is, for example, 0.5to 50 μm.

A mask pattern for ELO (Epitaxial Lateral Overgrowth) may be provided onthe surface of the single crystal GaN layer. The mask pattern is formedby a thin film made of a material that inhibits vapor phase growth ofGaN such as silicon nitride (SiN_(x)) and silicon oxide (SiO₂). Apreferred example of the mask pattern is a stripe pattern (line andspace pattern). The direction of the stripes is set to be parallel to anm-axis of the single crystal GaN layer.

Growth of a GaN crystal by an HVPE method can be performed using a vaporphase growth apparatus of which a conceptual diagram is shown in FIG. 5.

The vapor phase growth apparatus shown in FIG. 5 has a growth furnace100, inlet pipes 101 to 103 for introducing gas into the growth furnace,a reservoir 104 that retains metallic gallium, a heater 105 arranged soas to enclose the growth furnace, a susceptor 106 for mounting a seed,and an exhaust pipe 107 for discharging gas from inside the growthfurnace.

The growth furnace, the inlet pipes, the reservoir, and the exhaust pipeare formed of quartz. The susceptor 106 is formed of carbon and,favorably the surface thereof is coated by SiC.

Ammonia (NH₃), gallium chloride (GaCl), and a carrier gas are suppliedto the growth furnace 100 through the inlet pipes 101 to 103.

Gallium chloride is created by a reaction of hydrogen chloride (HCl)supplied to the reservoir 104 through the inlet pipe 102 with metallicgallium retained in the reservoir. Since the hydrogen chloride issupplied to the reservoir after being diluted by nitrogen gas (N₂), thegas introduced into the growth furnace through the reservoir containshydrogen chloride and nitrogen gas in addition to gallium chloride.

Hydrogen gas (H₂) and nitrogen gas (N₂) are favorably used as thecarrier gas.

A susceptor temperature during crystal growth may favorably be adjustedas appropriate within a range of 900 to 1200° C.

Internal pressure of the growth furnace during crystal growth mayfavorably be adjusted as appropriate within a range of 50 to 120 kPa.

The susceptor 106 is rotated so that a crystal grows uniformly on theseed. The rotation rate may be adjusted as appropriate between, forexample, 1 and 50 rpm.

The crystal growth rate is favorably set to a range of 80 to 300 μm/h.The growth rate may be increased by increasing partial pressure ofeither one or both of GaCl and ammonia in the growth furnace. Thepartial pressure of GaCl is favorably 2×10² to 2×10³ Pa. The partialpressure of ammonia is favorably 4×10³ to 1×10⁴ Pa.

Gas partial pressure as used herein means a value (P×r) obtained bymultiplying pressure (P) in a growth furnace by a ratio (r) of avolumetric flow rate of specific gas among a sum of volumetric flowrates of all gases supplied into the growth furnace.

The primary substrate (C-plane GaN substrate) is fabricated by slicing,parallel to the C-plane, the primary GaN crystal grown on the C-planeGaN template. Details of techniques necessary for planarizing theprincipal surface and removing a damage layer are well known to thoseskilled in the art and need not be particularly described. A nitrogenpolarity surface of the primary substrate is subjected to a CMP(Chemical Mechanical Polishing) finish for planarization and removal ofa damage layer. The nitrogen polarity surface is a [000-1] sideprincipal surface of a C-plane GaN substrate and is also called an Npolarity face, a nitrogen face, and the like.

(ii) Growth of Secondary GaN Crystal and Fabrication of SecondarySubstrate (M-Plane GaN Substrate)

The secondary GaN crystal to be used as a material of the secondarysubstrate is grown by an ammonothermal method using the primarysubstrate as a seed.

Before growing the secondary GaN crystal, a growth mask for limiting aregion enabling crystal growth is formed on the nitrogen polaritysurface of the primary substrate.

FIG. 6 is a schematic diagram exemplifying the primary substrate onwhich a growth mask is formed. The primary substrate 1001 has arectangular nitrogen polarity surface 1001 a, and arranged thereon is agrowth mask 1002 which has a linear opening with a width W_(o) of around50 to 100 μm and which has a stripe pattern (line and space pattern)parallel to an a-axis. A stripe period P_(s) is favorably set largerthan 1 mm and is favorably set equal to or smaller than 10 mm.

An end surface 1001 b in an a-axis direction and an end surface 1001 cin a m-axis direction of the primary substrate must not be covered bythe growth mask. In an example, furthermore, an outer peripheral portionof the nitrogen polarity surface may be exposed within a range ofseveral mm from a substrate end surface.

The growth mask is formed by a metal that does not dissolve or decomposeduring growth of a GaN crystal by an ammonothermal method such as Al, W,Mo, Ti, Pt, Ir, Ag, Au, Ta, Ru, Nb, and Pd, or an alloy thereof.

A raw material used in the ammonothermal method is favorablypolycrystalline GaN. Concentration of oxygen contained as an impurity inthe polycrystalline GaN is favorably 5×10¹⁹ cm⁻³ or lower.

An amount of impurities such as water and oxygen contained in ammoniaused as a solvent is favorably 0.1 ppm or less.

An acidic mineralizer is used as a mineralizer. A preferred example ofthe acidic mineralizer is an acidic mineralizer including a halogenelement such as ammonium halide, gallium halide, and hydrogen halide. Acombination of ammonium fluoride and hydrogen iodide is particularlyfavorable.

Growth of a GaN crystal by the ammonothermal method may be performedusing a high-pressure growth apparatus of which a conceptual diagram isshown in FIG. 7.

Crystal growth is carried out in a cylindrical growth vessel 20 that isloaded into a cylindrical autoclave 1.

The growth vessel 20 is internally provided with a crystal growth zone 6and a raw material dissolving zone 9 that are partitioned from eachother by a baffle 5. A seed crystal 7 suspended by a platinum wire 4 isinstalled in the crystal growth zone 6. The raw material dissolving zone9 is filled with raw material 8.

A gas line to which a vacuum pump 11, an ammonia cylinder 12, and anitrogen cylinder 13 are connected is connected to the autoclave 1 via avalve 10. When charging the growth vessel 20 with ammonia, an amount ofammonia supplied from the ammonia cylinder 12 can be checked using amass flow meter 14.

When growing a crystal, the growth vessel 20 having a seed, the rawmaterial, the mineralizer, and a solvent sealed therein is loaded intothe autoclave 1, a space between the autoclave 1 and the growth vessel20 is also filled with the solvent, and the autoclave 1 is hermeticallysealed. Subsequently, the whole autoclave 1 is heated by a heater (notshown) to create a supercritical state or a subcritical state inside thegrowth vessel 20.

During crystal growth, pressure inside the growth vessel 20 is favorablyset to 180 MPa or higher and 300 MPa or lower and a temperature thereinis favorably set to 530° C. or higher and 650° C. or lower. The rawmaterial dissolving zone 9 is kept at a higher temperature than thecrystal growth zone 6. A difference in temperature between the two zonesis favorably 80° C. or less.

FIG. 8 schematically shows how the secondary GaN crystal grows. In FIG.8, the secondary GaN crystal 1003 is growing in a wall shape one by oneabove each opening of the growth mask 1002. The height direction of thewalls is a [000-1] direction (−c direction) and the thickness directionof the walls is an m-axis direction. While each of the walls also growsin the thickness direction, coalescence of adjacent walls is unlikely tooccur.

Since an interface between the secondary GaN crystal 1003 and theprimary substrate 1001 is limited within the long and narrow openingprovided to the growth mask 1002, stress generated at the interface canbe prevented from affecting the growth of the secondary GaN crystal.

Although omitted in FIG. 8, since a GaN crystal also grows from endsurfaces of the primary substrate, a structure schematically shown inFIG. 9 is formed as a whole. A GaN crystal that grows from an endportion 1001 b in an a-axis direction of the primary substrate extendsin the [000-1] direction and forms a wall 1004 having an inclined outersurface. An end portion 1003 b of the secondary GaN crystal in an a-axisdirection connects to the inner surface of the wall 1004. A GaN crystalthat grows from an end portion 1001 c in an m-axis direction of theprimary substrate also extends in the [000-1] direction and forms a wall1005 having an inclined outer surface. The wall 1004 and the wall 1005connect to each other to form a peripheral wall structure enclosing thesecondary GaN crystal 1003.

Due to the formation of the structure shown in FIG. 9, at least thethree effects presented below are produced.

The first effect is an effect of holding the secondary GaN crystal onthe primary substrate.

The second effect is an effect of preventing the growth rate of thesecondary GaN crystal in the [000-1] direction from slowing down.

The third effect is an effect of preventing the size of the secondaryGaN crystal in an a-axis direction from decreasing.

The first effect enables regrowth in a growth process of the secondaryGaN crystal.

Regrowth means an operation in which, at a time when a crystal is grownto a certain degree, the seed is taken out from the growth vessel andplaced in a new growth vessel, and a crystal is once again grown on theseed. Since the growth rate declines as the raw material is consumed inthe growth vessel, regrowth is essential to obtaining a large-sizesecondary GaN crystal. The formation of the structure shown in FIG. 9enables such regrowth.

If the structure is not formed, it is difficult to transfer thesecondary GaN crystal from a used growth vessel to a new growth vesselwhile preventing the secondary GaN crystal from falling off from theprimary substrate. This is because, as described previously, directbonding between the secondary GaN crystal and the primary substrate islimited to inside of the long and narrow opening provided to the growthmask.

When the structure shown in FIG. 9 is formed, since the secondary GaNcrystal is fastened to the primary substrate via the peripheral wallstructure described above, in addition to enabling a regrowth operation,significantly reduced is the probability of detachment of the secondaryGaN crystal from the primary substrate in a growth vessel due to actionof convection of the solvent or the like.

Furthermore, the peripheral wall structure shown in FIG. 9 has an effectof protecting the secondary GaN crystal from suffering damage duringhandling.

The second effect described above may be explained by way of modeling asfollows.

For example, a case where a GaN crystal is grown using a GaN crystalseed substrate 2001 shown in FIG. 10 (a) will be considered. A principalsurface 2001 a of the seed substrate is a nitrogen polarity surface anda shape thereof is a long and narrow rectangle extending in an a-axisdirection.

A GaN crystal grows at a high rate on the nitrogen polarity surface ofthe seed substrate immediately after start of growth, but, at an earlystage, stable faces 2002 b and 2002 c appear on a surface of a GaNcrystal 2002 as shown in FIG. 10 (b). Since the stable faces 2002 b and2002 c are both inclined with respect to the [000-1] direction that is agrowth direction of the GaN crystal 2002, as crystal growth progresses,the nitrogen polarity surface 2002 a of the GaN crystal narrows.Eventually, as shown in FIG. 10 (c), when an entire surface of the GaNcrystal is occupied by the stable faces 2002 b and 2002 c and thenitrogen polarity surface 2002 a disappears, the growth rate of the GaNcrystal in the [000-1] direction drops to an impractical level.

In contrast, when the structure shown in FIG. 9 is formed, since astable face is less likely to appear on the surface of the secondary GaNcrystal, slowing of the growth rate in the [000-1] directionattributable to the disappearance of a nitrogen polarity surface doesnot occur. Therefore, by performing regrowth, the secondary GaN crystalcan be grown in the [000-1] direction to 15 mm or more, 20 mm or more,or even 25 mm or more while maintaining the growth rate in the [000-1]direction to a practical level (for example, higher than 100 μm/day).Regrowth can be repeated twice or more.

The third effect described above can also be explained by a comparisonwith a model shown in FIG. 10. In the model shown in FIG. 10, as the GaNcrystal 2002 grows in the [000-1] direction, the size thereof in ana-axis direction decreases. This is because the stable face 2002 b thatappears at an end portion in an a-axis direction is inclined withrespect to the c-axis.

In contrast, when the structure shown in FIG. 9 is formed, an endportion 1003 b of the secondary GaN crystal in an a-axis direction isjoined to the wall 1004. Therefore, a reduction in size of the secondaryGaN crystal in an a-axis direction due to the appearance of an inclinedstable face does not occur.

Accordingly, the size of the secondary GaN crystal in an a-axisdirection is generally determined in accordance with the size of theprimary substrate in an a-axis direction. The size of the primarysubstrate in an a-axis direction is dependent on the size of the C-planeGaN template that is used as a seed when growing the primary GaNcrystal. Therefore, for example, by fabricating the primary substratewith a diameter exceeding 2 inches from the primary GaN crystal grownusing a C-plane GaN template with a diameter of 3 inches and growing thesecondary GaN crystal using the primary substrate as a seed, thesecondary GaN crystal with a size in an a-axis direction of 2 inches (50mm) or more can be obtained.

The size (thickness) of the secondary GaN crystal in an m-axis directionreaches approximately 1 mm or more (restricted by the stripe periodP_(s) of the growth mask 1002).

By cutting off outer peripheral portions for shaping and planarizingboth principal surfaces by lapping and CMP, the secondary GaN crystalcan be made into the secondary substrate (M-plane GaN substrate).

To fabricate the secondary substrate with a size in a c-axis directionof 10 mm or more, the secondary GaN crystal that is used as a materialis desirably grown to 15 mm or more in a [000-1] direction. The presentinventors have confirmed that, by growing the secondary GaN crystalaccording to the method described above, the secondary substrate havinga 52 mm×52 mm rectangular principal surface can be fabricated. However,since the material of the secondary substrate is the secondary GaNcrystal, the secondary substrate has low heat resistance and is notsuitable as a substrate for epitaxially growing a nitride semiconductorby a vapor phase method such as a MOCVD method, an HVPE method, and thelike.

(iii) Growth of Tertiary GaN Crystal and Fabrication of TertiarySubstrate (M-Plane GaN Substrate)

The tertiary GaN crystal to be used as a material of the tertiarysubstrate is grown by an ammonothermal method using the secondarysubstrate as a seed.

Raw material, a solvent, and a mineralizer which may be favorably usedwhen growing the tertiary GaN crystal are the same as those used whengrowing the secondary GaN crystal. A usable crystal growth apparatus andfavorable crystal growth conditions are also the same as thoseapplicable to growing the secondary GaN crystal.

While a GaN crystal grows so as to cover an entire surface of thesecondary substrate, the portion of the GaN crystal which can befavorably used as a material for the tertiary substrate is an M-facegrown portion formed on a principal surface of the secondary substrate.

The use of an acidic mineralizer containing fluorine such as ammoniumfluoride is particularly recommended when growing the tertiary GaNcrystal. This mineralizer has an effect of significantly acceleratingthe rate of M-face growth of a GaN crystal. At present, no basicmineralizers enabling M-face growth of a GaN crystal at a practical ratehave been developed.

A GaN crystal ammonothermally grown using a fluorine-contained acidicmineralizer contains fluorine and concentration thereof normally exceeds1×10¹⁵ cm⁻³.

The tertiary substrate (M-plane GaN substrate) is fabricated by slicingthe tertiary GaN crystal parallel to an M-plane. The principal surfaceis planarized by lapping and/or grinding and then subjected to a CMPfinish to remove a damage layer.

The principal surface of the tertiary substrate is given a rectangularshape. Among four sides constituting the rectangle, two sides arearranged parallel to an a-axis and the other two sides are arrangedparallel to a c-axis. Names of end surfaces of the tertiary substrateare defined as shown in FIG. 11. Specifically, an end surface positionedat an end portion in an a-axis direction is called an A end surface. Inaddition, among end surfaces positioned at end portions in a c-axisdirection, an end surface on a +c side ([0001] side) is called a +C endsurface and an end surface on a −c side ([000-1] side) is called a −Cend surface. The +C end surface is a gallium polarity surface and the −Cend surface is a nitrogen polarity surface.

Improving machining accuracy of the tertiary substrate and reducingvariations of orientations of the principal surfaces and the endsurfaces are extremely important for arranging a plurality of thetertiary substrates so that crystal orientations are aligned in the nextstep.

Specifically, regarding the principal surface of the tertiary substrate,deviations from respective design values of an a-axis directioncomponent and a c-axis direction component of an off-angle are made tobe within ±0.2° and favorably within ±0.1°.

Similarly, regarding the orientations of the +C end surface and the −Cend surface, deviations from design values with respect to an a-axisdirection and a m-axis direction are respectively made to be within±0.2° and favorably within ±0.1°.

When necessary, deviations from design values of the orientation of theA end surface with respect to a c-axis direction and an m-axis directionare respectively made to be within ±0.2° and favorably within ±0.1°.

(iv) Growth of Quaternary GaN Crystal and Fabrication of Self-StandingGaN Substrate

The quaternary GaN crystal to be used as a material of the self-standingGaN substrate according to the first embodiment is grown by an HVPEmethod on an aggregated seed formed by arranging a plurality of tertiarysubstrates (M-plane GaN substrates).

FIG. 12(a) is a plan view of an aggregated seed formed by arranging fivetertiary substrates side by side so that principal surfaces thereof faceupward. The tertiary substrates are arranged in an c-axis direction and,between adjacent tertiary substrates, the +C end surface of one tertiarysubstrate and the −C end surface of the other tertiary substrate are incontact with each other.

By improving orientation accuracy of the principal surface, the +C endsurface, and the −C end surface of each tertiary substrate, crystalorientations of the plurality of tertiary substrates constituting theaggregated seed can be precisely matched with one another.

In an example, by arranging the tertiary substrates not only in a c-axisdirection but also in an a-axis direction, an upper surface of theaggregated seed may be further expanded. In this case, favorably,orientation accuracy of the A end surface of each tertiary substrate isalso improved.

Growth of a GaN crystal by an HVPE method can be performed using a vaporphase growth apparatus described previously of which a conceptualdiagram is shown in FIG. 5. However, in this step, all of orapproximately all of (99% or more) the carrier gas is to be constitutedby nitrogen gas.

FIG. 12(b) is a sectional view showing a state where a GaN crystal grownby an HVPE method has covered the aggregated seed shown in FIG. 12(a).As shown in FIG. 12(b), a GaN crystal that collectively covers fivetertiary substrates can be grown. In FIG. 12(b), in a GaN crystal grownby an HVPE method, portions grown above boundaries between adjacenttertiary substrates are shown hatched. These portions tend to have ahigher density of crystal defects such as dislocations and stackingfaults than other portions.

When growing the quaternary GaN crystal by an HVPE method, in order thata step-flow growth mode occurs as early as possible in an initial stage,a two-step growth method including (a) a temperature-raising step, (b) apreliminary growth step, and (c) a main growth step described below inthis order is favorably performed.

(a) Temperature-Raising Step

In the temperature-raising step, a susceptor temperature is raised fromroom temperature to T₁ without supplying gallium chloride to the seed.T₁ is favorably 830° C. or higher and 870° C. or lower. A temperatureraising rate is favorably 12° C./min or higher and 30° C./min or lower.The temperature raising rate may remain constant through the duration ofthe temperature-raising step or be changed during thetemperature-raising step.

An atmospheric gas that may be introduced into the growth furnace in thetemperature-raising step is hydrogen gas, ammonia, nitrogen gas, or thelike. Favorably, at least both ammonia and nitrogen gas are introduced.A volumetric flow rate of ammonia introduced into the growth furnace isfavorably 15% or more of a sum of volumetric flow rates of all gasesintroduced into the growth furnace.

(b) Preliminary Growth Step

In the preliminary growth step, while epitaxially growing a GaN crystalby supplying gallium chloride and ammonia to the seed, the susceptortemperature is raised from T₁ to T₂. T₂ is favorably 940° C. or higherand 1200° C. or lower. A temperature raising rate is favorably 6° C./minor higher and 24° C./min or lower.

When the pressure in the growth furnace in the preliminary growth stepis set to 1.0×10⁵ Pa, GaCl partial pressure is favorably 2.0×10² Pa orhigher and 5.0×10² Pa or lower and ammonia partial pressure is favorably9.3×10³ Pa or higher and 1.2×10⁴ Pa or lower.

In the preliminary growth step and the next main growth step, all of orapproximately all of (99% or more) the carrier gas supplied into thegrowth furnace is to be constituted by nitrogen gas.

(c) Main Growth Step

In the main growth step, gallium chloride and ammonia are supplied ontothe seed while maintaining the susceptor temperature at T₂ and a GaNcrystal is grown until a thick film is formed. The pressure in thegrowth furnace in the main growth step is favorably 50 kPa or higher and120 kPa or lower.

When the pressure in the growth furnace in the main growth step is setto 1.0×10⁵ Pa, Gad partial pressure is favorably 1.5×10² Pa or higherand 5.0×10² Pa or lower and ammonia partial pressure is favorably1.0×10³ Pa or higher and 1.2×10⁴ Pa or lower.

FIG. 13 shows an example of a profile of a susceptor temperatureadoptable in the two-step growth method described above. In thisexample, a temperature-holding step is provided between thetemperature-raising step and the preliminary growth step.

According to the two-step growth method, by starting crystal growth atthe temperature T₁ that is lower than the susceptor temperature T₂ inthe main growth step, a decomposition product of quartz constituting thegrowth furnace and the like is prevented from being adsorbed on a seedsurface before the start of the main growth step. The present inventorsconsider that a decomposition product of quartz is adsorbed on a seedsurface to reduce its wettability with GaN and delay the occurrence of astep-flow growth mode of GaN.

In the preliminary growth step and the main growth step, GaN can begrown while supplying doping gas.

As doping gas for oxygen doping, oxygen gas (0₂) or water (H₂O) can befavorably used. As doping gas for silicon doping, silane (SiH₄),disilane (Si₂H₆), chlorosilane (SiH₃Cl), dichlorosilane (SiH₂Cl₂),trichlorosilane (SiHCl₃), tetrachlorosilane (SiCl₄), tetrafluorosilane(SiF₄), or the like can be favorably used.

Instead of supplying doping gas, oxygen doping of a GaN crystal can alsobe performed using oxygen-containing gas that is generated bydecomposition of quartz constituting the growth furnace and the like. Asthe temperature of quartz members during crystal growth rises, an amountof generated oxygen-containing gas increases and oxygen concentration inthe GaN crystal rises. When the oxygen concentration in the GaN crystalmust be kept low, a method described in Japanese Patent ApplicationLaid-open No. 2012-066983 can be applied. Examples are as follows.

1) Arrange a liner tube made of high-purity pBN (pyrolytic boronnitride) inside a growth furnace, and by arranging a seed inside theliner tube, prevent oxygen-containing gas generated from the quartzgrowth furnace from coming to contact with the seed.2) In 1) above, by causing high-purity nitrogen gas to flow as shieldinggas between the growth furnace and the liner tube, the oxygen-containinggas generated from the quartz growth furnace can be more effectivelyprevented from coming to contact with the seed.3) Provide a susceptor on which a seed is to be mounted with a localheating mechanism, and heat the seed using the local heating mechanismand heaters in combination. Accordingly, since heater power necessary toheat the susceptor to a prescribed temperature can be reduced,temperatures of the growth furnace and the like that are heated togetherwith the seed by the heater can be lowered and, consequently, generationof oxygen-containing gas from these quartz members can be suppressed.4) By inhibiting transmission of heat from heaters to a reservoir usingmeans such as a thermal insulation plate, a temperature of the reservoircan be lowered and, consequently, an amount of oxygen-containing gasgenerated from the reservoir can be reduced.

A growth thickness of the quaternary GaN crystal is determined takinginto consideration of a surface orientation of a self-standing GaNsubstrate to be fabricated. The thicker the quaternary GaN crystal, aGaN substrate with a larger angle between a principal surface thereofand an M-plane can be cut out.

The seed is separated from the quaternary GaN crystal by slicing orremoved by grinding.

Types of processing performed when fabricating a self-standing GaNsubstrate from the quaternary GaN crystal are not limited. Necessaryprocessing in accordance with required specifications of the substratemay be performed including core drilling, slicing, grinding, lapping,etching, CMP, and beveling.

Favorably, at least one of the principal surfaces of the self-standingGaN substrate is planarized so that RMS roughness is reduced to lessthan 1 nm to ensure that the self-standing GaN substrate can befavorably used for epitaxial growth. Planarization is favorablyperformed by, but not limited to, CMP.

While a damage layer is favorably removed from the principal surfaces, amethod of removal is not limited and may be arbitrarily selected fromknown methods including CMP, dry etching, and wet etching.

<Second Method>

The self-standing GaN substrate according to the first embodiment canalso be manufactured using the tertiary GaN crystal as referred to inthe first method described above as a material.

Types of processing performed when fabricating the self-standing GaNsubstrate according to the first embodiment from the tertiary GaNcrystal are not limited. Necessary processing in accordance withrequired specifications of the substrate may be performed including coredrilling, slicing, grinding, lapping, etching, CMP, and beveling.

Because of using the tertiary GaN crystal as a material, theself-standing GaN substrate manufactured by the second method has goodheat resistance and may be preferably used as a substrate forepitaxially growing a nitride semiconductor by a vapor phase method suchas an MOCVD method and an HVPE method.

<Modified Method>

In the first method described above, the tertiary substrate is a M-planeGaN substrate.

In a modified method, when processing the tertiary GaN crystal tofabricate the tertiary substrate, an angle formed between a principalsurface of the tertiary substrate and an m-axis is set to a relativelylarge angle. Simply put, a semipolar substrate such as a (30-31)substrate, a (30-3-1) substrate, a (20-21) substrate, and a (20-2-1)substrate is fabricated as the tertiary substrate.

When the tertiary substrate is a (30-31) substrate, a quaternary GaNcrystal grows on the (30-31) substrate with a [30-31] direction as athickness direction. This quaternary GaN crystal may be processed tofabricate a self-standing GaN substrate according to the firstembodiment. While it is obvious that a (30-31) substrate and a (30-3-1)substrate may be fabricated, substrates that can be fabricated are notlimited thereto. By changing a slicing or polishing direction, aself-standing GaN substrate with various surface orientations such as a(20-21) substrate and an M-plane substrate can be fabricated from thequaternary GaN crystal.

2.2. Second Embodiment 2.2.1. Self-Standing GaN Substrate

A feature of a self-standing GaN substrate according to the secondembodiment is that an anomalous transmission image can be obtained bytransmission X-ray topography.

FIG. 14 is a diagram schematically showing an arrangement of an X-raysource, a test piece, and a detector in transmission X-ray topography byLang's method. An X-ray source is arranged on the side of one principalsurface of a plate-like test piece with a thickness of t, and an X-raydetector is arranged on the side of the other principal surface.

An anomalous transmission of an X-ray is also called a Borrmann effectand is a phenomenon where an X-ray is transmitted through a crystal thatis normally too thick for the x-ray to transmit due to absorption. Forexample, when a transmission image is obtained from a GaN substrate witha thickness of 344 μm in X-ray topography using MoKa (wavelength 0.71073Å) as an X-ray source, the image is an anomalous transmission image.This is because, an absorption coefficient μ of GaN is 290.40 (cm⁻¹)when the X-ray source is MoKa, therefore μt=10.0 when the thickness t ofa GaN substrate is 344 μm, and a transmission image cannot be obtainedunder a condition of μt≥10.0 unless an anomalous transmission occurs.

This does not mean that transmission X-ray topography under a conditionof μt≥10.0 is essential when distinguishing between a GaN substrateaccording to the second embodiment and other GaN substrates. A GaNsubstrate according to the second embodiment may be the one for which ananomalous transmission is observed in a transmission X-ray topographicimage acquired under a condition of μt<10.0.

Since an anomalous transmission is not observed when the perfection of acrystal is low, the fact that an anomalous transmission image isobtained in X-ray topography proves that the quality of the crystal isextremely high. While X-ray topographic analysis using anomaloustransmission is already being performed with respect to single crystalsof Si and GaAs [for example, refer to J. R. Patel, Journal of AppliedPhysics, Vol. 44, pp. 3903-3906 (1973) or P. Mock, Journal of CrystalGrowth, Vol. 224, pp. 11-20 (2001)], to the best of the knowledge of thepresent inventors, cases where X-ray anomalous transmission is observedhave not yet been reported with respect to a GaN crystal.

Favorably, with the self-standing GaN substrate according to the secondembodiment, the size of the projected image when a principal surfacethereof is vertically projected on an M-plane is a 10 mm square or more.

Favorably, with the self-standing GaN substrate according to the secondembodiment, when the principal surface thereof is vertically projectedon the M-plane, the size of the projected image in a c-axis direction is15 mm or more and the size of the projected image in an a-axis directionis 25 mm or more.

When producing a self-standing GaN substrate for which an anomaloustransmission image is obtained by X-ray topography, it is favorable toprovide an inspection process including transmission X-ray topographyusing anomalous transmission as a test item. By deeming a product inwhich an unacceptable defect is found in the inspection process adefective product, only products with particularly preferable crystalquality can be shipped.

2.2.2. Manufacturing Method of Self-Standing GaN Substrate

The present inventors have confirmed that the self-standing GaNsubstrate according to the second embodiment can be fabricated using themanufacturing method described previously in 2.1.2. In other words, theself-standing GaN substrate according to the second embodiment may be aself-standing GaN substrate constituted by a GaN crystal grown by anHVPE method.

Otherwise, the self-standing GaN substrate according to the secondembodiment can be fabricated from the tertiary GaN crystal as referredto in the manufacturing method of a self-standing GaN substratedescribed previously in 2.1.2.

2.3. Third Embodiment 2.3.1. Self-Standing GaN Substrate

A feature of a self-standing GaN substrate according to the thirdembodiment is that a dislocation density in an effective region of theprincipal surface is less than 4×10⁵ cm⁻². The dislocation density in aneffective region of the principal surface as described herein means adislocation density obtained by dividing a total number of dislocationsexisting in the effective region by an area of the effective region, theeffective region being a region excluding portions at a distance of 2 mmor less from a substrate end surface, of the principal surface. Thereason why portions at a distance of 2 mm or less from a substrate endsurface are removed is because an outer peripheral portion of asubstrate is likely to retain defects attributable to mechanicalprocessing of a crystal. Normally, in order to eliminate the effect ofremaining defects, a region excluding an outer peripheral portion isused to form a semiconductor device. It is needless to say that,preferably, the outer peripheral portion also has a low defect density.

In a self-standing GaN substrate according to the third embodiment, thedislocation density in the effective region of the principal surface isfavorably less than 1×10⁵ cm⁻² and more favorably less than 4×10⁴ cm⁻².

Dislocations on the principal surface of a GaN substrate can be observedas dark spots by a usual cathode luminescence (CL) method. Therefore,the dislocation density as used herein can be described as a density ofdark spots observed by a CL method.

With a GaN substrate having high parallelism between the principalsurface and an M-plane, by etching the principal surface with heatedsulfuric acid, an etch pit with a size that is even detectable by anoptical microscope can be formed at locations where dislocations exist.Typical etching conditions include a sulfuric acid concentration of 85to 90 wt %, a temperature of 265 to 275° C., and an etching time of 45to 90 minutes. The present inventors have confirmed that a density ofetch pits formed by the heated sulfuric acid etching described above hasa higher degree of coincidence with a dark spot density obtained by CLmeasurement as compared to other methods. The number of dislocationsexisting on the principal surface of a substrate can also be obtained bycounting the number of etch pits. In this case, using an opticalmicroscope is convenient because a wide field of view can be observed.

Favorably, with a self-standing GaN substrate according to the thirdembodiment, the size of the projected image when the principal surfacethereof is vertically projected on an M-plane is a 10 mm square or more.

Favorably, with a self-standing GaN substrate according to the thirdembodiment, when the principal surface thereof is vertically projectedon an M-plane, the size of the projected image in a c-axis direction is15 mm or more and the size of the projected image in an a-axis directionis 25 mm or more.

2.3.2. Manufacturing Method of Self-Standing GaN Substrate

A material of the self-standing GaN substrate according to the thirdembodiment is, for example, the tertiary GaN crystal as referred to inthe manufacturing method of a self-standing GaN substrate describedpreviously in 2.1.2.

While there are variations in dislocation density of a self-standing GaNsubstrate obtained by processing the tertiary GaN crystal, even whensuch variations are taken into consideration, the dislocation density inthe effective region of the principal surface rarely equals or exceeds4×10⁵ cm⁻². Although portions with high dislocation density and portionswith low dislocation density are often formed on the principal surfacewith the substrate whose dislocation density is high, in such a case, anaverage dislocation density in a 200 μm×200 μm region contained in theportion with high dislocation density is around 4×10⁵ cm⁻² at themaximum.

Although high yield is yet to be achieved, the substrate with adislocation density in the effective region of the principal surface of1×10⁴ cm⁻² has been produced. From what is known to date, for example,the smaller an off-angle of a principal surface of a seed used whengrowing the tertiary GaN crystal or, in other words, the higher theparallelism between the principal surface of the seed and an M-plane,the lower the dislocation density of the tertiary GaN crystal.

There are variations in crystallinity even in a single piece of thetertiary GaN crystal. For example, a portion near an outermost surfacetends to have a higher dislocation density than a portion near a seed(the secondary substrate described previously). Therefore, in order tofabricate a self-standing substrate with a lower dislocation density, aportion relatively near the seed in the tertiary GaN crystal isdesirably used as a material. However, there is a case where a portionadjacent to the seed has low heat resistance.

Types of processing performed when fabricating the self-standing GaNsubstrate according to the third embodiment from the tertiary GaNcrystal are not limited. Necessary processing in accordance withrequired specifications of the substrate may be performed including coredrilling, slicing, grinding, lapping, etching, CMP, and beveling.

2.4. Fourth Embodiment 2.4.1. Self-Standing GaN Substrate

A feature of a self-standing GaN substrate according to the fourthembodiment is that a stacking fault density in an effective region ofthe principal surface is less than 15 cm⁻¹. The stacking fault densityin an effective region of the principal surface as described hereinmeans a stacking fault density obtained by dividing a total length ofstacking faults existing in an effective region by an area of theeffective region, the effective region being a region excluding portionsat a distance of 2 mm or less from a substrate end surface, of theprincipal surface.

In the self-standing GaN substrate according to the fourth embodiment,the stacking fault density in the effective region of the principalsurface is favorably less than 5 cm⁻¹ and more favorably less than 1cm⁻¹.

A total length of stacking faults on a principal surface of a GaNsubstrate can be obtained by growing a GaN thin film by an MOCVD methodon the principal surface and observing a surface of the thin film by anoptical microscope. For example, the GaN thin film may be grown to athickness of 2 μm using nitrogen gas as carrier gas and under conditionsincluding an ammonia flow rate of 10 slm, a trimethyl gallium supplyrate of 206 μmol/min, pressure of 12.5 kPa, and a substrate temperatureof 1040° C.

Since a step is formed in correspondence to a stacking fault on thesurface of the GaN thin film, a length of the stacking fault can beobtained by measuring a length of the step by optical microscopeobservation.

Favorably, with the self-standing GaN substrate according to the fourthembodiment, the size of the projected image when the principal surfacethereof is vertically projected on an M-plane is a 10 mm square or more.

Favorably, with the self-standing GaN substrate according to the fourthembodiment, when the principal surface thereof is vertically projectedon an M-plane, the size of the projected image in a c-axis direction is15 mm or more and the size of the projected image in an a-axis directionis 25 mm or more.

2.4.2. Manufacturing Method of Self-Standing GaN Substrate

A material of the self-standing GaN substrate according to the fourthembodiment is, for example, the tertiary GaN crystal as referred to inthe manufacturing method of a self-standing GaN substrate describedpreviously in 2.1.2.

With a GaN crystal, the lower the parallelism between a growth directionthereof and a c-axis, the higher the likelihood of an occurrence of astacking fault. This tendency is not limited to vapor phase growth andsimilarly applies to an ammonothermal method.

However, because of being grown on a seed (secondary substrate)fabricated from the secondary GaN crystal with an extremely low level ofdistortion, the tertiary GaN crystal has a very low stacking faultdensity despite the parallelism between the growth direction and thec-axis being low.

While there are variations in stacking fault density of a self-standingGaN substrate obtained by processing the tertiary GaN crystal, even whensuch variations are taken into consideration, the stacking fault densityin the effective region of the principal surface rarely equals orexceeds 15 cm⁻¹.

Although high yield is yet to be achieved, an M-plane substrate with astacking fault density in the effective region of the principal surfaceof 0.05 cm⁻¹ has been produced.

Even a single tertiary GaN crystal has variations in crystallinity. Forexample, a portion near an outermost surface tends to have a higherstacking fault density than a portion near a seed (the secondarysubstrate described earlier). Therefore, in order to fabricate aself-standing substrate with fewer stacking faults, a portion relativelynear the seed in the tertiary GaN crystal is desirably used as amaterial. However, there is a case where a portion adjacent to the seedhas low heat resistance.

Types of processing performed when fabricating the self-standing GaNsubstrate according to the fourth embodiment from the tertiary GaNcrystal are not limited. Necessary processing in accordance withrequired specifications of the substrate may be performed including coredrilling, slicing, grinding, lapping, etching, CMP, and beveling.

3. Use of Self-Standing GaN Substrate 3.1. Seed Crystal

The self-standing GaN substrate according to the present invention maybe used as a seed for epitaxially growing a nitride semiconductor.

In an example, a GaN single crystal may be obtained by epitaxiallygrowing GaN by an arbitrary method on the self-standing GaN substrateaccording to the present invention. This GaN single crystal may be abulk single crystal.

In another example, a first GaN single crystal may be fabricated byepitaxially growing GaN using the self-standing GaN substrate accordingto the present invention as a seed and, subsequently, a second GaNsingle crystal may be fabricated by epitaxially growing GaN using a partof or all of the first GaN single crystal as a seed. This second GaNsingle crystal may be a bulk single crystal.

In yet another example, by growing a bulk GaN single crystal by anammonothermal method or the like using the self-standing GaN substrateaccording to the present invention as a seed and subsequently slicingthe bulk GaN single crystal including the seed, a GaN substrateincluding a seed portion can be fabricated.

3.2. Semiconductor Device

The self-standing GaN substrate according to the present invention mayalso be used to manufacture a semiconductor device. Normally, one ormore types of nitride semiconductors are epitaxially grown on theself-standing GaN substrate according to the present invention to form adevice structure. As an epitaxial growth method, an MOVCD method, an MBEmethod, a pulsed deposition method, or the like suitable for forming athin film may be favorably used.

Specific examples of a semiconductor device include a light-emittingdevice such as a light-emitting diode and a laser diode, an electronicdevice such as a rectifier, a bipolar transistor, a field effecttransistor, and an HEMT (High Electron Mobility Transistor), asemiconductor sensor such as a temperature sensor, a pressure sensor, aradiation sensor, and a visible-ultraviolet light detector, and thelike.

Furthermore, the self-standing GaN substrate according to the presentinvention may also be applied to use including a SAW (Surface AcousticWave) device, a vibrator, a resonator, an oscillator, a MEMS (MicroElectro Mechanical System) component, a voltage actuator, and anelectrode for an artificial photosynthesis device.

3.3. GaN Layer-Bonded Substrate

A GaN layer-bonded substrate may be manufactured using the self-standingGaN substrate according to the present invention as a material.

A GaN layer-bonded substrate refers to a composite substrate in which aGaN layer is bonded to a hetero-composition substrate with a differentchemical composition from GaN and can be used to manufacture alight-emitting device and other semiconductor devices.

Typically, a GaN layer-bonded substrate is manufactured by executing, inthis order, the steps of: implanting ions in a vicinity of a principalsurface of a GaN substrate; bonding the principal surface side of theGaN substrate to a hetero-composition substrate; and forming a GaN layerbonded to the hetero-composition substrate by separating the GaNsubstrate at the ion-implanted region as a boundary.

Therefore, when the self-standing GaN substrate according to the presentinvention is used as a material, a GaN layer-bonded substrate isobtained having a structure in which a GaN layer separated from theself-standing GaN substrate according to the present invention is bondedto a hetero-composition substrate.

When used as a material of a GaN layer-bonded substrate, an initialthickness of the self-standing GaN substrate according to the presentinvention may be 1 mm or more, 2 mm or more, or 4 mm or more.

Examples of hetero-composition substrates that may be used tomanufacture a GaN layer-bonded substrate include a sapphire substrate,an AlN substrate, a SiC substrate, a ZnSe substrate, a Si substrate, aZnO substrate, a ZnS substrate, a quartz substrate, a spinel substrate,a carbon substrate, a diamond substrate, a Ga₂O₃ substrate, a ZrB₂substrate, a Mo substrate, and a W substrate.

Details of a structure, a manufacturing method, use, and the like of aGaN layer-bonded substrate are described in Japanese Patent ApplicationLaid-open No. 2006-210660, Japanese Patent Application Laid-open No.2011-44665, and the like.

4. Experimental Results 4.1. Experiment 1 4.1.1. Fabrication ofSelf-Standing GaN Substrate

[1] Growth of primary GaN crystal and fabrication of primary substrate

A GaN crystal (primary GaN crystal) was grown by an HVPE method on asapphire-based C-plane GaN template with a striped mask pattern for ELOformed on a principal surface thereof. Next, a C-plane GaN substrate(primary substrate) with a rectangular principal surface having verticaland horizontal sides respectively parallel to an a-axis and an m-axiswas fabricated from the GaN crystal. A nitrogen polarity surface of theC-plane GaN substrate was made into a flat surface with no damage layerby performing a CMP finish.

[2] Growth of Secondary GaN Crystal and Fabrication of SecondarySubstrate

A growth mask with a stripe pattern having 100 μm-wide linear openingsat a period of 1100 μm was formed with a TiW alloy on the nitrogenpolarity surface of the primary substrate fabricated in [1] above. Alongitudinal direction of the openings or, in other words, a stripedirection was arranged parallel to the a-axis.

After forming the growth mask, a GaN crystal (secondary GaN crystal) wasgrown by an ammonothermal method using the primary substrate as a seed.Polycrystalline GaN manufactured by a method of causing a vapor phasereaction of NH₃ and GaCl was used as a raw material and ammoniumfluoride (NH₄F) and hydrogen iodide (HI) were used as mineralizers.

Charge amounts of NH₄F and HI were determined so that a molar ratio offluorine atoms to NH₃ was 0.5 to 1.5%, a molar ratio of iodine atoms toNH₃ was 1.5 to 3.5% and, a molar ratio of fluorine atoms to iodine atomswas 0.2 to 0.5.

Growth conditions included setting an average temperature inside agrowth vessel (an average value of temperatures in a crystal growth zoneand a raw material dissolving zone) to 590 to 615° C., a difference intemperatures between the crystal growth zone and the raw materialdissolving zone to 10 to 25° C., and pressure inside the growth vesselto 200 to 220 MPa. A growth period was set to a total of 58 days or moreexcluding time required for replacing growth vessels for regrowth.

A structure shown in FIG. 9 was formed by the GaN crystalammonothermally grown on the primary substrate. From a portion grown ina wall shape on each opening of the growth mask in the grown GaN crystal(secondary GaN crystal), an M-plane GaN substrate (secondary substrate)with a length of 50 to 54 mm, a width of 8 to 11 mm, and a thickness of280 to 320 μm and having a rectangular principal surface with a longside parallel to an a-axis and a short side parallel to a c-axis wasfabricated. Both principal surfaces of the secondary substrate weresubjected to a CMP finish.

[3] Growth of Tertiary GaN Crystal and Fabrication of Tertiary Substrate

A GaN crystal (tertiary GaN crystal) was further grown by anammonothermal method using the secondary substrate fabricated in [2]above as a seed.

For this second ammonothermal growth, the charge amount of themineralizer were set such that molar ratios of fluorine atoms and iodineatoms to NH₃ were respectively 0.5% and 1.5%. In addition, the averagetemperature inside the growth vessel was set to 600 to 611° C., thedifference in temperatures between the crystal growth zone and the rawmaterial dissolving zone was set to 9 to 13° C., and the growth periodwas set to 15 days or more.

From the tertiary GaN crystal obtained as a result of the secondammonothermal growth, an M-plane GaN substrate (tertiary substrate) witha length of 41 to 59 mm, a width of 5 to 11 mm, and a thickness of 330μm and having a rectangular principal surface with a long side parallelto an a-axis and a short side parallel to a c-axis was fabricated.

Among the two principal surfaces of the tertiary substrate, a side to beused for epitaxial growth of a quaternary GaN crystal in a next step(front surface) was given an off-angle of −2° in a direction. Theaccuracies of a [0001] direction component and a [11-20] directioncomponent of the off-angle were respectively set to ±0.1°.

Furthermore, four end surfaces of the tertiary substrate were alsoformed at similar accuracy. In other words, a +C end surface and a −Cend surface were arranged parallel to a C-plane and respectively giveninclinations within ±0.1° in a [10-10] direction and a [11-20]direction. A end surfaces were arranged parallel to an A-plane andrespectively given inclinations within ±0.1° in a [0001] direction and a[10-10] direction.

In order to ensure the accuracies described above, for each end surface,a procedure was performed involving confirming an orientation using anX-ray diffractometer every time the GaN crystal was cut by a dicing sawfor forming an end surface, and when a deviation from a designorientation was outside a permissible range, adjusting a direction ofthe work and once again cutting the work.

The principal surfaces of the tertiary substrate were finished by a CMPprocess.

With respect to the front surface of the tertiary substrate, observationof basal plane dislocations by a normal temperature cathode luminescencemethod (SEM-CL method: an acceleration voltage of 3 kV, a beam currentof 100 pA, and an observation area of 0.09 mm×0.12 mm) and observationof stacking faults by a low temperature cathode luminescence method(SEM-CL method: an acceleration voltage of 10 kV, a beam current of 4nA, an observation area of 0.45 mm×0.59 mm, and a sample temperature of82 K) were performed. For both measurements, 7 to 10 points (5 mmintervals) on a straight line parallel to the a-axis were selected asmeasurement points. However, neither basal plane dislocations norstacking faults were detected.

[4] Growth of Quaternary GaN Crystal and Fabrication of Self-StandingM-Plane GaN Substrate

A GaN crystal was grown by an HVPE method using an aggregated seedprepared by aligning seven tertiary substrates fabricated in [3] aboveside by side in a c-axis direction with their front surfaces facingupward. The seven tertiary substrates were closely aligned so that,between two adjacent substrates, the +C end surface of one substratecame into contact with the −C end surface of the other substrate.

When growing the GaN crystal, the two-step growth method describedearlier was used. In other words, at first, while only supplyingnitrogen gas and ammonia to the seed, a susceptor temperature was raisedto 850° C. (temperature-raising step) and the temperature was maintainedfor 5 seconds (temperature-holding step).

Next, by supplying hydrogen chloride diluted by nitrogen gas to areservoir holding metallic gallium and heated to 800° C., supply ofgallium chloride to the seed was started and, at the same time, thesusceptor temperature was raised at a rate of 6.5° C./minute(preliminary growth step).

After the susceptor temperature had reached 1060° C., gallium chlorideand ammonia were supplied onto the seed while keeping the temperatureconstant to grow GaN for 97 hours (main growth step).

After the start of the temperature-raising step until the end of themain growth step, pressure inside the growth furnace was controlled at1.0×10⁵ Pa, GaCl partial pressure was controlled at 2.1×10² Pa, andammonia partial pressure was controlled at 5.7×10³ Pa. Carrier gassupplied into the growth furnace was entirely constituted by nitrogengas.

The grown GaN crystal collectively covered the seven tertiary substratesand the thickness thereof was 6.8 mm at a central portion of the seed.The thickness of the GaN crystal decreased from the central portiontoward an outer peripheral portion of the aggregated seed and was 4.5 mmat the portion near the +c side ([0001] side) end and 5.5 to 6.2 mm atthe portion near the −c side ([000-1] side) end.

From a portion of the GaN crystal at a distance of approximately 1 mmfrom a seed surface, a self-standing M-plane GaN substrate with a lengthof 50 mm, a width of 25 mm, and a thickness of 346 μm and having arectangular principal surface with a long side parallel to an a-axis anda short side parallel to a c-axis was cut out. The principal surface onthe X-ray incident side in the X-ray analysis performed later was givenan off-angle of −2.0°±0.1° in a [0001] direction and 0.0°±0.1° in a[−12-10] direction. The principal surface was first planarized bylapping and then subjected to a CMP finish for further planarization andremoval of a damage layer.

4.1.2. Evaluation of Self-Standing GaN Substrate

[1] A-Axis Length and c-Axis Length

In order to study a distortion distribution in the self-standing GaNsubstrate fabricated in 4.1.1 [4] above, an lattice spacing measurementwas performed using a high-resolution X-ray diffractometer [PANalyticalX′ Pert Pro MRD manufactured by Spectris Co., Ltd.].

In the X-ray diffraction measurement, using an optical system includinga divergence slit, a mirror for increasing beam parallelism, and a Ge(220) asymmetric 2-bounce monochromator, only a CuKα1 line was extractedand used from a line-focused X-ray beam emitted by an X-ray tube. A beamshape was adjusted using a pinhole collimator so that a full-width athalf-maximum of a Gaussian function approximation on a sample surfacewas 100 μm in a horizontal direction and 400 μm in a vertical direction.

The M-plane GaN substrate was fixed to a sample stage so that the c-axisdirection was horizontal and the a-axis direction was vertical. Then,X-rays were made incident to the principal surface of the sample toperform a 2θ-ω scan of a (300) plane and a (20-3) plane every 250 μm ona straight line parallel to a c-axis direction.

When performing the 2θ-ω scan, a Ge (220) 3-bounce monochromator (aso-called analyzer) and a proportional counter detector were used on alight-receiving side. To prevent temperature fluctuations from affectingthe measurements, the temperature inside a housing of the X-raydiffractometer was controlled within 24.5±1° C. The origin of 2θ wascalibrated upon start of measurement and checked for deviations at theend of the measurement.

With respect to lattice spacing, XRD analysis software [PANalyticalX'Pert Epitaxy] manufactured by Spectris Co., Ltd. was used to fit aspectrum of the 2θ-ω scan by a Gaussian function to obtain a peak, andthe lattice spacing was derived by a calculation based on dynamicaltheory from the value of the peak.

Measurements of lattice spacing were performed with respect to the (300)plane and the (20-3) plane. The (300) plane was selected because,compared to a (100) plane, 2θ is measured at a higher angle side and,consequently, a measurement with higher angular resolution can beperformed.

An a-axis length was obtained from the (300) lattice spacing and ac-axis length was calculated based on the (300) lattice spacing and the(20-3) lattice spacing.

The a-axis length was obtained by expression (1) below from the (300)lattice spacing.

a=2√3×d ₍₃₀₀₎×(1+α₁)  (1)

The respective symbols used in expression (1) signify the following:

a: a-axis length [Å]

α₁: 2.52724×10⁻⁵ (correction coefficient)

d₍₃₀₀₎: (300) lattice spacing (measured value) [Å]

The c-axis length was obtained by expressions (2) and (3) below from the(300) lattice spacing and the (20-3) lattice spacing.

$\begin{matrix}{c = \frac{3}{\sqrt{\left( \frac{1}{M} \right)^{2} - \left( \frac{4}{\sqrt{3} \times a} \right)^{2}}}} & (2)\end{matrix}$M=d ₍₂₀₋₃₎×(1+α₂)  (3)

The respective symbols used in expressions (2) and (3) signify thefollowing:

c: c-axis length [Å]

α₂: 1.10357×10⁻⁴ (correction coefficient)

d₍₂₀₋₂₎: (20-3) lattice spacing (measured value) [Å]

α₁ in expression (1) and α₂ in expression (3) are, respectively,correction coefficients for refraction correction of the values of thea-axis length and the c-axis length. The correction coefficients werecalculated by referring to “The Study of Imperfections in SemiconductorSingle Crystals by Precise Measurements of Lattice Parameters” byYasumasa Okada in “Researches of Electrotechnical Laboratory” Volume 913(June 1990) (ISSN: 0366-9106) and using expression (4) below.

$\begin{matrix}{\alpha = \frac{\left( {4.48 \times 10^{- 7}} \right) \times n_{o}\lambda^{2}\cos \; \mu}{{\sin \left( {\theta + \mu} \right)} \cdot {\sin \left( {\theta - \mu} \right)}}} & (4)\end{matrix}$

The respective symbols used in expression (4) signify the following:

n₀: the number of electrons per 1 nm³ of crystal

λ: wavelength of X-ray

θ: angle of incidence

μ: angle between crystal surface and diffraction plane

Results of measurements of the a-axis length and the c-axis length onthe principal surface of the M-plane GaN substrate fabricated in 4.1.1[4] above taken every 250 μm on a straight line along a c-axis directionare respectively shown in FIGS. 15 and 16.

As shown in FIG. 15, a variation width of the a-axis length was 4.9×10⁻⁵Å in all sections excluding two exceptional sections. The exceptionalsections were a section with a c-axis direction position of −6.5 mm to−6 mm and a section with a c-axis direction position of 2.25 mm to 3.5mm. The exceptional sections corresponded to portions constituted by GaNcrystals having grown above boundaries between adjacent tertiarysubstrates and exhibited a large local variation in the a-axis length.Lengths of both exceptional sections were less than 1.5 mm.

As shown in FIG. 16, a variation width of the c-axis length was 1.8×10⁻⁴Å in all sections excluding end portions and two exceptional sections.The exceptional sections were a section with a c-axis direction positionof −6.5 mm to −6 mm and a section with a c-axis direction position of2.25 mm to 3.5 mm. The exceptional sections corresponded to portionsconstituted by GaN crystals having grown above boundaries betweenadjacent tertiary substrates and exhibited a large local variation inthe c-axis length. Lengths of both exceptional sections were less than 2mm.

A relationship between a variation width of the a-axis length and avariation width of (300) lattice spacing will now be described. Sincethe relationship between the a-axis length (after refraction correction)and (300) lattice spacing (measured value) is as represented byexpression (1) above, a variation in the a-axis length of 10.0×10⁻⁵ Å orless means that a variation width of (300) lattice spacing is less than2.9×10⁻⁵ Å.

[2] X-Ray Rocking Curve

With respect to the M-plane GaN substrate used in the lattice spacingmeasurement of 4.1.2. [1] above (the M-plane GaN substrate fabricated in4.1.1. [4] above), X-ray rocking curves of a (300) plane and a (030)plane were measured every 250 μm on a straight line parallel to thec-axis using the same high-resolution X-ray diffractometer as used inthe lattice spacing measurement.

While the result of the measurement of X-ray rocking curve of the (300)plane showed that co was discontinuous between GaN crystals respectivelygrown on two adjacent tertiary substrates, discontinuity Δω was onlyaround 0.01°.

In addition, while the result of the measurement of X-ray rocking curveof the (030) plane also showed that w was discontinuous between GaNcrystals respectively grown on two adjacent tertiary substrates,discontinuity Δω was only 0.015°.

These results signify that, when observed from a GaN crystal grown on agiven tertiary substrate, an orientation of a GaN crystal formed on anadjacent tertiary substrate is only inclined by about 0.01° around thec-axis as a center of rotation and only inclined by about 0.015° aroundthe m-axis as a center of rotation.

This is conceivably a manifestation of the effect of improvingorientation accuracies of the principal surface, the +C end surface, andthe −C end surface of the tertiary substrates.

[3] X-Ray Rocking Curve Full-Width at Half-Maximum (XRC-FWHM)

With respect to the M-plane GaN substrate used in the lattice spacingmeasurement of 4.1.2. [1] above (the M-plane GaN substrate fabricated in4.1.1. [4] above), an X-ray rocking curve full-width at half-maximum(XRC-FWHM) of a (300) plane on the principal surface was measured every250 μM on a straight line parallel to the c-axis using the samehigh-resolution X-ray diffractometer as used in the lattice spacingmeasurement. Results thereof are shown in FIG. 17.

As shown in FIG. 17, the XRC-FWHM was less than 85 arcsec excluding twoexceptional sections. The exceptional sections were a section with ac-axis direction position of −7.25 mm to −5.5 mm and a section with ac-axis direction position of 3.25 mm to 4 mm. In these exceptionalsections, a variation in the XRC-FWHM increased and spots where thevalue of the XRC-FWHM was prominently high were observed. Theseexceptional sections corresponded to portions constituted by GaNcrystals having grown above boundaries between adjacent tertiarysubstrates.

A variation width of the XRC-FWHM was less than 10 arcsec excluding thetwo exceptional sections.

[4] Reflection X-Ray Topography

Using the high-resolution X-ray diffractometer described above, areflection X-ray topographic image of the M-plane GaN substrate used inthe lattice spacing measurement of 4.1.2. [1] above (the M-plane GaNsubstrate fabricated in 4.1.1. [4] above) was acquired.

For the X-ray topography, a long line-focus tube with a Cu target wasused in a line focus mode, and an incident optical system including adivergence slit, a 50 μm mask, and a mirror for increasing beamparallelism was used. In a light-receiving system, a semiconductortwo-dimensional detector with an element size of 55×55 μm and a detectorsize of 14 mm×14 mm was used.

FIG. 18 shows a reflection X-ray topographic image obtained using (203)diffraction of a portion (a portion not including a crystal grown abovea boundary between adjacent tertiary substrates) of the M-plane GaNsubstrate.

A striped pattern in a horizontal direction (c-axis direction) is seenin the image shown in FIG. 18. This striped pattern does not represent adistortion of the crystal. Instead, since there is periodicity in thea-axis direction, it is understood that the striped pattern representsan intensity variation of diffraction rays appeared due to an intensityvariation in incident X-rays.

A noteworthy point is that, across a wide range of approximately 20 mmin an a-axis direction and 6 mm or more in a c-axis direction, thedensity of the striped pattern is uniform or, in other words, theintensity variation of diffraction rays reflects only the intensityvariation of the incident X-rays. This means that a crystal latticestructure of the GaN crystal constituting the substrate is uniform notonly in a c-axis direction but also in an a-axis direction. Based on theX-ray topograph, it was presumed that the measurements of latticespacing and rocking curves described above will produce approximatelythe same result even when carried out at different positions.

In order to substantiate the presumption, with respect to an M-plane GaNsubstrate separately fabricated by the same method as Experiment 1, thepresent inventors measured an a-axis length on the principal surface ontwo parallel straight lines. The two straight lines were respectivelyparallel to the c-axis and spacing (a distance in an a-axis direction)between the two straight lines was 5 mm. The measurement was performedevery 250 μm on each straight line. As a result, a low distortionsection with an a-axis length variation of less than 5×10⁻⁵ Å wasobserved over 10 to 11 mm on both straight lines.

4.2. Experiment 2

An M-plane GaN substrate was fabricated by the same procedure asExperiment 1 above with the exception of changing the carrier gas usedwhen growing a GaN crystal by an HVPE method on a seed constituted bytertiary substrates.

More specifically, compared to all of the carrier gas being constitutedby nitrogen gas when growing a GaN crystal by the HVPE method in 4.1.1.[4] above, in Experiment 2, 44% of a volumetric flow rate of the carriergas was constituted by hydrogen gas and the remainder by nitrogen gas.

Results of measurements of an a-axis length and a c-axis length takenevery 250 μm along a c-axis direction using the high-resolution X-raydiffractometer described above with respect to the fabricated M-planeGaN substrate are shown in FIGS. 19 and 20.

In addition, FIG. 21 shows a reflection X-ray topographic image obtainedusing (203) diffraction of a portion (a portion not including a crystalgrown above a boundary between adjacent tertiary substrates) of a GaNsubstrate of a comparative example fabricated in Experiment 2.

It is understood from FIGS. 19 and 20 that, despite grown using seeds ofsimilar quality, the M-plane GaN substrate according to Experiment 2 haslarger variations in the a-axis length and the c-axis length and greaterdisturbance of crystal as compared to the M-plane GaN substrateaccording to Experiment 1 described previously.

A reflection X-ray topographic image shown in FIG. 21 exhibits extremelysharp contrast between light and dark portions and has extremely lowuniformity of diffraction intensity. This indicates that the crystal issignificantly distorted.

4.3. Experiment 3 [1] Experiment 3-1

A self-standing M-plane GaN substrate was fabricated in a similar mannerto Experiment 1 described earlier. However, in Experiment 3-1, the sizein a c-axis direction of the secondary GaN crystal fabricated by thefirst ammonothermal growth was enlarged and, accordingly, sizes of thesecondary substrate and the tertiary substrate in a c-axis directionwere enlarged.

Results of measurements of an a-axis length and a c-axis length takenevery 250 μm along a c-axis direction using the high-resolution X-raydiffractometer described above with respect to the obtained M-planeself-standing GaN substrate are shown in FIGS. 22 and 23.

From FIGS. 22 and 23, it is understood that the M-plane GaN substrateaccording to Experiment 3-1 has a large crystal disturbance at an endportion whose c-axis direction position exceeds 11 mm. Upon use, thisportion is favorably removed.

Other than the end portion whose c-axis direction position exceeds 11mm, variation widths of the a-axis length and the c-axis length wererespectively 5.9×10⁻⁵ Å and 1.9×10⁻⁴ Å excluding two exceptionalsections.

The exceptional sections were a section with a c-axis direction positionof −6.75 mm to −5.75 mm and a section with a c-axis direction positionof 5.5 mm to 6.25 mm. These sections corresponded to portionsconstituted by GaN crystals having grown above boundaries betweenadjacent tertiary substrates and exhibited large local variations in thea-axis length and the c-axis length. Lengths of both exceptionalsections were less than 1.5 mm.

[2] Experiment 3-2

A self-standing M-plane GaN substrate was fabricated in a similar mannerto Experiment 1 described previously. An external view photograph of theself-standing GaN substrate is shown in FIG. 24 (a).

Although the substrate is shown to be cracked in the external viewphotograph, this was due to inappropriate handling. The substrate had athickness of 240 μm and did not crack when handled appropriately.

A transmission X-ray topographic image was acquired using one of thepieces of the self-standing M-plane GaN substrate shown in FIG. 24(a) asa sample. The X-RAY Topographic Imaging System XRT-300 by RigakuCorporation was used for X-ray topography. MoKa was used as an X-raysource and an imaging plate with a pixel size of 50 μm was used as adetector. As result, a transmission X-ray topograph shown in FIG. 24 (b)was obtained using (002) diffraction.

Due to the fact that a transmission topographic image was obtained undera condition of μt=7 where transmission of X-rays is difficult to occurwithout contribution of anomalous transmission, it was found that notonly the surface but the entire self-standing M-plane GaN substrateconsisted of GaN crystal with exceptionally high quality.

4.4. Experiment 4

A self-standing M-plane GaN substrate was fabricated in a similar mannerto Experiment 1 above with the exception of changing the carrier gasused when growing a GaN crystal by an HVPE method on a seed constitutedby the tertiary substrate.

Specifically, in Experiment 4, when growing a GaN crystal by the HVPEmethod, 92% of a volumetric flow rate of the carrier gas was constitutedby nitrogen gas and 8% by hydrogen gas.

Results of measurements of an a-axis length and a c-axis length takenevery 250 μm along a c-axis direction using the high-resolution X-raydiffractometer described above with respect to the obtained M-plane GaNsubstrate are shown in FIGS. 25 and 26.

FIGS. 25 and 26 show that variations in the a-axis length and the c-axislength are relatively large on a straight line parallel to the c-axis onthe principal surface of the M-plane GaN substrate of Experiment 4.

4.5. Experiment 5 [1] Experiment 5-1

A self-standing M-plane GaN substrate with a length of 26 mm, a width of17 mm, and a thickness of 470 μm and having a rectangular principalsurface with a long side parallel to an a-axis and a short side parallelto a c-axis was fabricated by a method substantially the same as thatused for the tertiary substrate fabricated in 4.1.1. [3] above.

The M-plane GaN substrate was annealed by placing in an atmospherecontaining nitrogen gas and ammonia at a volumetric ratio of 9:1 at1000° C. for 50 hours.

After the annealing, oxides adhered to the surface were removed and,furthermore, both principal surfaces were subjected to lapping and CMP.The size of the substrate after these processings was 26 mm in an a-axisdirection, 16 mm in a c-axis direction, and 350 μm in an m-axisdirection.

Next, the M-plane GaN substrate was further cut to obtain a principalsurface size of 12 mm×12 mm to fabricate a test piece for X-raytopography. FIG. 27 (a) shows an external view photograph of the testpiece.

Results of X-ray topography of the test piece are shown in FIGS. 27 (b)to 27(d).

An X-ray topography apparatus (product name: XRT micron) by RigakuCorporation was used for X-ray topography. MoKa was used as an X-raysource and an X-ray CCD camera with a pixel size of 5.4 μm was used as adetector.

FIG. 27 (b) is a transmission X-ray topograph of the test piece obtainedusing (002) diffraction. In addition, FIG. 27(c) is a transmission X-raytopograph of the test piece obtained using (110) diffraction. Sinceμt=10.2, it is understood that these images were formed by anomaloustransmission.

FIG. 27(d) is a reflection X-ray topograph of the same test pieceobtained using (203) diffraction. As shown in the diagram, thereflection topograph exhibited extremely high uniformity over the entire12 mm×12 mm region. Accordingly, it is conceivable that the light andshade observed in the anomalous transmission X-ray topograph reflectdistortion or defects existing inside the substrate.

[2] Experiment 5-2

A 350 μm-thick self-standing M-plane GaN substrate with a longitudinaldirection parallel to an a-axis was fabricated by a method substantiallythe same as that used for the tertiary substrate fabricated in 4.1.1.[3] above. Annealing was not performed in Experiment 5-2.

FIG. 28(a) shows an external view photograph of the fabricatedsubstrate. The size of the principal surface was 35 mm (total length) inan a-axis direction and 15 mm in a c-axis direction.

FIG. 28(b) is a transmission X-ray topograph of the self-standingM-plane GaN substrate obtained using (002) diffraction. The X-RAYTopographic Imaging System XRT-300 by Rigaku Corporation was used forthe measurement. MoKα was used as an X-ray source and an imaging platewith a pixel size of 50 μm was used as a detector.

Since μt=10.2, it is understood that this transmission X-ray topographicimage was formed by anomalous transmission.

[3] Experiment 5-3

The a-axis length on a principal surface of the self-standing M-planeGaN substrate of Experiment 5-2 was measured every 250 μm on straightlines parallel to the c-axis using the high-resolution X-raydiffractometer described previously. As schematically shown in FIG. 29,measurements were respectively performed on five straight lines A to Earranged at 5 mm intervals in an a-axis direction.

Measurement results on the straight lines A to E are shown in FIGS. 30to 34 in an order from A to E. On all of the straight lines, a variationwidth of the a-axis length was 8.0×10⁻⁵ Å or less with the exception ofend portions.

Furthermore, the a-axis length on the principal surface of theself-standing M-plane GaN substrate of Experiment 5-2 was measured every250 μm on a straight line parallel to the a-axis using thehigh-resolution X-ray diffractometer described previously. Resultsthereof are shown in FIG. 35.

[4] Experiment 5-4

A self-standing M-plane GaN substrate having a rectangular principalsurface was fabricated using, as a material, a GaN crystal fabricated bya method substantially the same as that used for the tertiary GaNcrystal fabricated in 4.1.1. [3] above. The principal surface had a sizeof 26 mm in an a-axis direction and 15 mm in a c-axis direction.

On the principal surface of the M-plane GaN substrate, an a-axis lengthand a c-axis length were measured every 250 μm along a c-axis directionusing the high-resolution X-ray diffractometer described previously. Asa result, as shown in FIG. 36, the a-axis length had a variation widthof 5.2×10⁻⁵ Å with the exception of end portions. The c-axis length hada variation width of 1.9×10⁻⁴ Å as shown in FIG. 37.

[5] Experiment 5-5

A self-standing M-plane GaN substrate having a rectangular principalsurface was fabricated by a method substantially the same as that usedfor the tertiary substrate fabricated in 4.1.1. [3] above. The principalsurface had a size of 16 mm in an a-axis direction and 10 mm in a c-axisdirection. A region excluding portions at a distance of 2 mm or lessfrom a substrate end surface, of the principal surface, was assumed tobe an effective region, and the total number of dislocations existing inthe effective region was measured. Then, the total number ofdislocations was divided by an area of the effective region to obtain adislocation density of 1.3×10⁴ cm⁻². In addition, a stacking faultdensity obtained by dividing a total length of stacking faults existingin the effective region by the area of the effective region was 0.05cm⁻¹.

4.6. Experiment 6

A GaN crystal fabricated by a method substantially the same as that usedfor the tertiary GaN crystal fabricated in 4.1.1. [3] above was slicedparallel to (30-31) to fabricate a self-standing GaN substrate with bothprincipal surfaces parallel to (30-31).

A GaN crystal was further grown by an ammonothermal method using theself-standing GaN substrate as a seed.

Using the GaN crystal obtained by this third ammonothermal growth as amaterial, a self-standing GaN (30-3-1) substrate with a rectangularprincipal surface was fabricated. The principal surface had a size of 21to 29 mm in an a-axis direction and a size of 10 to 18 mm in a directionperpendicular to the a-axis.

With respect to a plurality of the fabricated GaN (30-3-1) substrates, aregion excluding portions at a distance of 2 mm or less from a substrateend surface, of the principal surface, was assumed to be an effectiveregion and the total number of dislocations existing in the effectiveregion was measured. Then, the total number of dislocations was dividedby an area of the effective region to obtain a dislocation density. As aresult, the dislocation density of the substrate with a lowestdislocation density was 9.0×10² cm⁻² and the dislocation density of thesubstrate with a highest dislocation density was 6.3×10⁴ cm⁻².

In addition, a GaN thin film was grown on the principal surface by anMOCVD method and a surface of the thin film was observed by an opticalmicroscope to investigate lengths of stacking faults existing in theeffective region. Subsequently, the total value of the lengths of thestacking faults was divided by an area of the effective region to obtaina stacking fault density. As a result, the stacking fault density of thesubstrate with a lowest stacking fault density was 0 cm⁻¹ and thestacking fault density of the substrate with a highest stacking faultdensity was 1 cm⁻¹.

An a-axis length and a c-axis length on the principal surface of theself-standing GaN (30-3-1) substrate were measured every 250 μm on anintersection line between the principal surface and an A-plane using thehigh-resolution X-ray diffractometer described previously. Results arerespectively shown in FIGS. 38 and 39.

The a-axis length had a variation width of 5.5×10⁻⁵ Å. The c-axis lengthhad a variation width of 1.9×10⁻⁴ Å with the exception of end portions.

4.7. Experiment 7

A GaN crystal fabricated by a method substantially the same as that usedfor the tertiary GaN crystal fabricated in 4.1.1. [3] above was slicedparallel to (20-21) to fabricate a self-standing GaN substrate with bothprincipal surfaces parallel to (20-21).

A GaN crystal was further grown by an ammonothermal method using theself-standing GaN substrate as a seed.

Using the GaN crystal obtained by this third ammonothermal growth as amaterial, a self-standing GaN (20-21) substrate with a rectangularprincipal surface was fabricated. The principal surface had a size of 23to 26 mm in an a-axis direction and a size of 9 to 12 mm in a directionperpendicular to the a-axis.

With respect to a plurality of the fabricated GaN (20-21) substrates, aregion excluding portions at a distance of 2 mm or less from a substrateend surface, of the principal surface, was assumed to be an effectiveregion and the total number of dislocations existing in the effectiveregion was measured. Then, the total number of dislocations was dividedby an area of the effective region to obtain a dislocation density. As aresult, the dislocation density of the substrate with a lowestdislocation density was 4.0×10⁴ cm⁻² and the dislocation density of thesubstrate with a highest dislocation density was 1.6×10⁵ cm⁻². Inaddition, a GaN thin film was grown on the principal surface by an MOCVDmethod and a surface of the thin film was observed by an opticalmicroscope to investigate lengths of stacking faults existing in theeffective region. Subsequently, the total value of the lengths of thestacking faults was divided by an area of the effective region to obtaina stacking fault density. As a result, the stacking fault density of thesubstrate with a lowest stacking fault density was 0 cm⁻¹ and thestacking fault density of the substrate with a highest stacking faultdensity was 1 cm⁻¹.

An a-axis length and a c-axis length on the principal surface of theself-standing (20-21) GaN substrate were measured every 250 μm on anintersection line between the principal surface and an A-plane using thehigh-resolution X-ray diffractometer described previously. Results arerespectively shown in FIGS. 40 and 41.

From FIGS. 40 and 41, it is understood that the (20-21) GaN substrate ofExperiment 7 has a large crystal disturbance at an end portion whoseposition in a c-axis direction is less than −2 mm.

Other than this portion, the a-axis length had a variation width of5.9×10⁻⁵ Å and the c-axis length had a variation width of 1.7×10⁻⁴ Å.

This suggests that, in order to grow a GaN crystal with small crystaldisturbance by an ammonothermal method, a seed substrate with a smallangle between the normal of the principal surface and an m-axis may beadvantageously used.

While the present invention has been described in its embodiments, suchdescription is for illustrative purposes only and is not intended tolimit the present invention.

APPENDIX

Inventions derived from the inventions described above will now beappended.

(1e) A GaN single crystal product, wherein an anomalous transmissionimage is obtained by transmission X-ray topography. Examples include awafer product and an X-ray optical element product.(2e) The GaN single crystal product according to (1e) above, wherein theGaN single crystal product is a wafer product.(3e) The GaN single crystal product according to (1e) or (2e) above,wherein the GaN single crystal product comprises a GaN crystal grown byan HVPE method.(4e) The GaN single crystal product according to any one of (1e) to (3e)above, wherein an absorption coefficient at a wavelength of 450 nm is 2cm⁻¹ or less.(5e) The GaN single crystal product according to any one of (1e) to (4e)above, wherein the GaN single crystal product contains fluorine.(6e) The GaN single crystal product according to any one of (1e) to (5e)above, wherein alkali metal concentration is lower than 1×10¹⁵ cm⁻³.(7e) The GaN single crystal product according to any one of (1e) to (6e)above, wherein the GaN single crystal product contains a stacking fault.(8e) A crystal comprising GaN, processing of which enables fabricationof the GaN single crystal product according to any one of (1e) to (7e)above.(9e) A method of producing the GaN single crystal product according toany one of (1e) to (7e) above, the method having an inspection stepincluding transmission X-ray topography using anomalous transmission asa test item, wherein a product in which an unacceptable defect is foundin the inspection step is deemed a defective product.(10e) A manufacturing method of a GaN single crystal, includingpreparing the GaN single crystal product according to (2e) above andepitaxially growing GaN on the GaN single crystal product.(11e) A manufacturing method of a GaN single crystal, including growinga first GaN crystal using the GaN single crystal product according to(2e) above as a seed, and subsequently growing a second GaN crystalusing a part of or all of the first GaN crystal as a seed.(12e) The manufacturing method according to (10e) or (11e), wherein themanufacturing method is a manufacturing method of a bulk GaN singlecrystal.(13e) A manufacturing method of a semiconductor device, includingpreparing the GaN single crystal product according to (2e) above andforming a device structure by epitaxially growing one or more types ofnitride semiconductors on the GaN single crystal product.

CLAIM SCOPE AND INCORPORATED BY REFERENCE

While the present invention has been described with reference tospecific embodiments thereof, it will be understood by those skilled inthe art that various changes and modifications may be made withoutdeparting from the spirit and scope of the invention. In addition, it isto be understood that the entire contents of the specification, claims,and drawings of the basic application are incorporated by reference intothe present specification.

EXPLANATION OF REFERENCE NUMERALS

-   1 Autoclave-   4 Platinum wire-   5 Baffle-   6 Crystal growth zone-   7 Seed crystal-   8 Raw material-   9 Raw material dissolving zone-   10 Valve-   11 Vacuum pump-   12 Ammonia cylinder-   13 Nitrogen cylinder-   14 Mass flow meter-   20 Growth vessel-   100 Growth furnace-   101 to 103 Inlet pipe-   104 Reservoir-   105 Heater-   106 Susceptor-   107 Exhaust pipe-   1001 Primary substrate-   1002 Growth mask-   1003 Secondary GaN crystal

1: A self-standing GaN substrate with an angle between the normal of theprincipal surface and an in-axis of 0 degrees or more and 20 degrees orless, wherein: the size of the projected image in a c-axis directionwhen the principal surface is vertically projected on an M-plane is 10mm or more; and when a region excluding a portion at a distance of 2 mmor less from a substrate end surface, of the principal surface, isassumed to be an effective region, a stacking fault density obtained bydividing a total length of stacking faults existing in the effectiveregion by an area of the effective region is less than 15 cm⁻¹. 2: Theself-standing GaN substrate according to claim 1, wherein the size ofthe projected image is a 10 mm square or more. 3: The self-standing GaNsubstrate according to claim 1, wherein the size of the projected imagein a c-axis direction is 15 mm or more. 4: The self-standing GaNsubstrate according to claim 1, wherein the size of the projected imagein an a-axis direction is 25 mm or more. 5: The self-standing GaNsubstrate according to claim 1, wherein the self-standing GaN substratecontains fluorine. 6: The self-standing GaN substrate according to claim5, wherein a concentration of fluorine exceeds 1×10¹⁵ cm⁻³. 7: Theself-standing GaN substrate according to claim 1, wherein theself-standing GaN substrate contains a stacking fault. 8: Theself-standing GaN substrate according to claim 1, wherein the stackingfault density is less than 5 cm⁻¹. 9: The self-standing GaN substrateaccording to claim 8, wherein the stacking fault density is less than 110: The self-standing GaN substrate according to claim 1, wherein adislocation density obtained by dividing a total number of dislocationsexisting in the effective region by an area of the effective region isless than 4×10⁵ cm⁻². 11: The self-standing GaN substrate according toclaim 10, wherein the dislocation density is less than 1×10⁵ cm⁻². 12:The self-standing GaN substrate according to claim 11, wherein thedislocation density is less than 4×10⁴ cm⁻². 13: A manufacturing methodof a GaN single crystal, the method comprising: preparing theself-standing GaN substrate according to claim 1; and epitaxiallygrowing GaN on the self-standing GaN substrate. 14: A manufacturingmethod of a GaN single crystal, the method comprising: growing a firstGaN crystal using the self-standing GaN substrate according to claim 1as a seed; and subsequently, growing a second GaN crystal using a partof or all of the first GaN crystal as a seed. 15: The manufacturingmethod according to claim wherein the manufacturing method is amanufacturing method of a bulk GaN single crystal. 16: A manufacturingmethod of a semiconductor device, the method comprising: preparing theself-standing GaN substrate according to claim 1; and forming a devicestructure by epitaxially growing one or more types of nitridesemiconductors on the self-standing GaN substrate. 17: A manufacturingmethod of a GaN layer-bonded substrate method comprising: implantingions in a vicinity of the principal surface of the self-standing GaNsubstrate according to claim 1; bonding the principal surface side ofthe self-standing GaN substrate to a hetero-composition substrate; andforming a GaN layer bonded to the hetero-composition substrate byseparating the self-standing GaN substrate at the ion-implanted regionas a boundary. 18: A GaN layer-bonded substrate with a structure inwhich a GaN layer separated from the self-standing GaN substrateaccording to claim 1 is bonded to a hetero-composition substrate.